the Chromebooks share their file system to Linux using the 9p file
system, which does not support setting file ownership. Newly PrusaSlicer
will detect platform and it will not panick if copy_file() cannot set
file ownership after copying. It just logs the incident, and on
chromebooks the loglevel for that incident is "Info", not "Error".
Adjusted the full screen mode to contain menu bar.
Moved Platform.cpp/hpp to libslic3r
Namely, on ChromeOS virgl flips red/blue channels at least on some computers with multi-sampling enabled.
It seems it is sufficient to disable multi-sampling after the OpenGL context is created.
