There is a corner case where one side of a quad needs splitting and the other
side has only one segment. Previously this would produce either gaps or after
recent changes to stitch together geometry, uninitialized memory.
Now solve this by splitting into triangular patches, as suggested in the
DiagSplit paper. These triangular patches can be further subdivided themselves.
Dicing has special cases for 1 or 2 segments on edges. For more segments it
works the same as: quad dicing: A regular inner triangle grid stitched to the
outer edges.
Fix#136973: Inconsistent results with adaptive subdivision
Pull Request: https://projects.blender.org/blender/blender/pulls/139062
59b4842117