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.
7d4b3f2992