WIP: Add native Wayland support for Linux (#13197)

* Add runtime display backend detection for Wayland support

Add LinuxDisplayBackend utility to detect X11 vs Wayland at runtime
using GDK_IS_X11_DISPLAY / GDK_IS_WAYLAND_DISPLAY macros. This is
the foundation for removing the forced GDK_BACKEND=x11 and enabling
native Wayland support.

- New files: LinuxDisplayBackend.hpp/.cpp with get_linux_display_backend(),
  is_running_on_wayland(), and is_running_on_x11()
- Propagate wxHAVE_GDK_X11 / wxHAVE_GDK_WAYLAND from FindGTK3.cmake
  as compile definitions to libslic3r_gui
- No-op on non-Linux platforms (returns Unknown / false)

* Fix Phase 1 code quality: pragma once, source ordering, static cache

* Make X11 initialization conditional for Wayland support

Remove the unconditional GDK_BACKEND=x11 force that blocked native
Wayland. Replace with conditional logic:

- EGL safety fallback: re-force X11 only when wxUSE_GLCANVAS_EGL is
  off and WAYLAND_DISPLAY is set, with a warning log
- XInitThreads() only called when DISPLAY is set (X11 in use)
- __GLX_VENDOR_LIBRARY_NAME only set when DISPLAY is present (GLX-specific)
- WEBKIT_DISABLE_COMPOSITING_MODE only set under XWayland (both
  DISPLAY and WAYLAND_DISPLAY present)
- Guard X11/Xlib.h include with __has_include for robustness
- Restore display validation to accept either DISPLAY or WAYLAND_DISPLAY

This is Phase 2 of the Wayland support plan.

* Fix Phase 2: safer EGL macro check, add clarifying comments

* Add GLAD2 library and replace GLEW linkage in build system

Set up GLAD2 as a static library to replace GLEW for OpenGL loading.
GLAD2 supports both GLX and EGL, which is required for Wayland support.

- Create src/glad/ with pre-generated GLAD2 sources (GL 4.6 compat)
- Add src/glad/CMakeLists.txt building glad as a static library
- Wire glad into src/CMakeLists.txt before libvgcode
- Modify libvgcode to use shared glad for GL path (keeps local copy
  only for GLES2/Emscripten) to avoid duplicate symbol conflicts
- Replace GLEW::GLEW with glad in libslic3r_gui link libraries

Note: GLEW is kept in deps for OpenCSG. Code migration from GL/glew.h
to glad/gl.h headers will follow in Phase 3B+3C.

* Fix Phase 3A+3D: libvgcode GLAD include, dead files, dlopen dep, OpenGL link var

* Migrate from GLEW to GLAD: replace headers and API calls across codebase

Replace all #include <GL/glew.h> with <glad/gl.h> across 49 source files.
Migrate GLEW API calls to GLAD equivalents:
- glewInit/glewExperimental -> gladLoaderLoadGL()
- GLEW_EXT_* / GLEW_ARB_* extension checks -> GLAD_GL_EXT_* / GLAD_GL_ARB_*
- Remove GLEW-specific EGL/GLX mismatch #error guards (not needed with GLAD)
- Replace unavailable EXT symbols with core GL equivalents in
  GLCanvas3D.cpp (GL_MAX_SAMPLES, glRenderbufferStorageMultisample,
  glBlitFramebuffer, GL_READ/DRAW_FRAMEBUFFER)
- Update log messages from glewInit to gladLoadGL

* Fix Phase 3B+3C: remove GLEW find, clean EXT symbols, update attribution

- Remove find_package(GLEW) block from root CMakeLists.txt since GLEW
  is no longer linked by any main application code
- Remove "glew" from SLIC3R_STATIC option description
- Replace all remaining EXT framebuffer symbols with core equivalents
  in render_thumbnail_framebuffer_ext and _rectangular_selection_picking_pass
- Update AboutDialog credits from GLEW to GLAD

* Enable EGL in wxWidgets and add runtime GLX/EGL selection for Wayland

- Set wxUSE_GLCANVAS_EGL=ON in wxWidgets build and Flatpak manifest
- Add PreferGLX() call on X11 sessions for driver compatibility
- Remove Phase 2 safety fallback (EGL is now always compiled in)
- Guard SwapBuffers against hidden canvases to prevent Wayland stalls

* Fix Phase 4: move PreferGLX to app startup, fix FPS counter guard

Move wxGLCanvas::PreferGLX() from OpenGLManager::create_wxglcanvas()
(static initializer) to GUI_App::on_init_inner() before any wxGLCanvas
is constructed. This prevents a race where SkipPartCanvas could trigger
wxGLBackend::Init() before the GLX preference is set. The new location
also adds explicit is_running_on_wayland() detection with a warning for
unknown backends.

Move increment_fps_counter() inside the IsShownOnScreen() guard so FPS
is only counted when a frame is actually swapped.

* Update GLFW from 3.3.7 to 3.4 for runtime Wayland/X11 backend selection

Replace the compile-time GLFW_USE_WAYLAND flag (which locked to a single
backend) with GLFW 3.4's GLFW_BUILD_WAYLAND + GLFW_BUILD_X11 flags that
build both backends and auto-select at runtime based on the available
display server. This enables the CLI thumbnail renderer to work on both
Wayland and X11 sessions without separate builds.

* wayland: Fix UI call sites that rely on global screen coordinates

On Wayland, wxGetMousePosition() returns (0,0) and SetPosition() is a
no-op for top-level windows. Fix the highest-impact call sites:

- GLCanvas3D: Use cached m_mouse.position from event handlers instead
  of wxGetMousePosition() + ScreenToClient() in get_local_mouse_position()
- Plater: Use event-relative coords via ClientToScreen(e.GetPosition())
  instead of wxGetMousePosition() in 3 leave-window handlers
- BBLTopbar: Use event.GetPosition() and FindToolByPosition() directly
  in mouse handlers instead of wxGetMousePosition()/FindToolByCurrentPosition()
- Search: Use focus-based dismiss logic on Wayland instead of
  wxGetMousePosition()-based rect checks in SearchDialog and
  SearchObjectDialog
- GUI_App: Skip SetPosition() in window_pos_restore() on Wayland where
  it is a no-op; still restore size and maximize state
- Button: Position tooltip relative to button widget via ClientToScreen
  instead of wxGetMousePosition()

* Fix SearchDialog Wayland dismiss: guard against search_line focus

* flatpak: Add Wayland socket permission for native Wayland support

* spec

* Fix crash on Wayland when wxWidgets lacks EGL support

Restore the safety fallback that forces GDK_BACKEND=x11 when wxWidgets
was not built with wxUSE_GLCANVAS_EGL=ON. Without this, the GLX backend
tries to access a non-existent X11 display on native Wayland, crashing
in wxGLCanvas::IsDisplaySupported() with SIGSEGV at offset 0xe4.

Also add a defense-in-depth guard in detect_multisample() that skips
the IsDisplaySupported call entirely on Wayland without EGL.

Root cause: deps/wxWidgets must be rebuilt after enabling EGL. The
compile-time check in OrcaSlicer.cpp detects the mismatch and falls
back safely.

* Fix EGL detection: use wxHAS_EGL instead of wxUSE_GLCANVAS_EGL

wxUSE_GLCANVAS_EGL is a CMake build option, NOT a C++ preprocessor
macro. The actual macro defined in wxWidgets setup.h is wxHAS_EGL.
All compile-time EGL checks were using the wrong macro, causing
the safety fallback to always trigger even with a properly built
EGL-enabled wxWidgets.

* Fix GL function pointers invalidated on Wayland/EGL

gladLoaderLoadGL() dlopen's libGL.so.1 to resolve GL function pointers
via dlsym, then immediately dlclose's the handle. On X11/GLX this is
fine because the GLX context keeps libGL.so mapped. On Wayland/EGL,
nothing else holds libGL.so open, so dlclose unmaps it and all function
pointers become dangling — causing SIGSEGV on the first GL call.

Fix: on Wayland, use gladLoadGL(eglGetProcAddress) which resolves
function pointers through the EGL loader without opening/closing
libGL.so.

* fix crash on start and various rendering issues

* fix crash on close

* small refactor

* move GPU selection to desktop file

* clean up a bit

* clean up more

* fix appimage error
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