CCPR: Move flatness checks from geometry shaders to CPU

Vertex shaders can't discard geometry.

Bug: skia:
Change-Id: Iec22a103b7a4eda9a59fd99e48644183b0880f5f
Reviewed-on: https://skia-review.googlesource.com/82260
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
10 files changed