The reverse uv sampling operation generally fails when there the sample position
is within more than one uv triangle. Unfortunately, the combination of limited
floating point accuracy and very small triangle areas caused this case
sometimes, even if the triangles don't actually overlap. This happens relatively
rarely, but if it does, it causes annoying artifacts. The solution is to only
detect these kinds of overlapping triangles if they have a certain minimum size
and to be more relaxed otherwise.
Pull Request: https://projects.blender.org/blender/blender/pulls/120408
Jacques Lucke
f771f83029