Fixes Solid infill where there should be none #6482
Also the safety offsetting was revised to be enabled only where needed,
the "do safety offset" is now easy to discover by
a new ApplySafetyOffset::Yes enum, and safety offset over union, which
is better done by offset() / offset_ex() has been replaced with
new union_safety_offset() / union_safety_offset_ex() functions, which
better convey their meaning and which could be better optimized than
union() with the safety offset applied.
- Separate the 3 bands -- dangling, sloping and full overhanging -- regions and handle them with different support force deficits.
- Use a heuristic for overhanging edges to increase the number of support points generated for them
- Try to make overhangs and slopes deficit depend on stable area.
- removed define USE_AABB_INDIRECT (which switched between old and new AABB implementation)
- removed define SLIC3R_SLA_NEEDS_WINDTREE (relied on igl and was not used anyway)
- new define SLIC3R_HOLE_RAYCASTER (hides currently unused code)
- slight include cleanup
- removed obsolete source file SupportTreeIGL.cpp
