Refactor middle-out tessellation shader for fixed count

Refactors the middle-out shader in a way that will allow the fixed
count tessellator to easily run Wang's formula and "throw away"
unnecessary triangles by making them empty.

Bug: skia:10419
Change-Id: I86b76cb24ef43ccdbc890f7dfffcda91f8b01a9c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/416542
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed