Refactor CCPR coverage shaders for a vertex impl

Decouples geometry generation and analytic coverage. This paves the
way for a vertex shader implementation.

TBR=egdaniel@google.com

Bug: skia:
Change-Id: I2a183401bfe70b9f14b9b1cf035de6020a2135fa
Reviewed-on: https://skia-review.googlesource.com/60103
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
19 files changed