If there was a control point at an extreme position when drawing a curve
profile (in the bottom corner), the fill's trianglulation could fail, giving
a misleading view of the curve. This is because the extra points added to
create a closed shape were exactly on the border of the view.
This commit adds a small margin to those points, so the triangulation
doesn't fail because the line overlaps itself.
Another possible solution is to use a different algorithm to fill
the polygon, such as scanfill, which is used by curve objects.
This seemed simpler, and seems to work fairly robustly.
Differential Revision: https://developer.blender.org/D9989
5a498e6a7b