When drawing a curve profile (ex: for a custom bevel profile), if there was a control point at extreme positions (the bottom corner, generally), extra triangles would be rendered, 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 adds a small margin to those points, so we always render correctly.

Another solution suggested by Hans Goudey (HooglyBoogly) is to use a different algorithm to fill the points, such as scanfill, which is used by curve objects. This seemed simpler, and seems to work fairly robustly.