Revert "Revert "Support combining per-vertex coverage with color in default GP vertex shader""

This reverts commit ded032c4c989d3ab6fe74db351cdba858aca04ea.

Bug: skia:
Change-Id: I89a0ef492c1930c9ba248fc0a92bc054f2f17205
Reviewed-on: https://skia-review.googlesource.com/c/181840
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed