Fix #11489 (crash with force point inside polygon in centroid fill style)

This isn't actually a very good fix. The issue in the "maptopixel" simplification
is still there, it is just less obvious, while sacrificing a bit of QgsGeometry correctness
(like the fact that linear ring should have >= 4 points)

Along the way I have added some comments that may help others decode why the code does things it does.