The hard eraser changes end caps to "flat" when cutting strokes. This
requires detecting when the start or end points of a stroke have been
cut.
The previous method relied on checking the next curve's start point for
cuts to determine end point cuts, but this only holds true for cutting
strokes in the middle. When only the start point is cut the preceding
curve should not be affected.
The new test uses local properties of the `PointTransferData` to detect
unchanged source curve end points, instead of doing a look-ahead for the
next curve. This works using the source point indices, which are
inverted for end points. Combined with checking for the `is_cut` flag
this gives a reliable answer to when a stroke end point is cut.
Pull Request: https://projects.blender.org/blender/blender/pulls/119151
645aeb9dbe