Add Usage::kColor to SkVertices custom attributes

Always treated as unpremul, RGB(A) sRGB colors. Automatically
transformed to destination color space, and premuled.

Bug: skia:9984
Change-Id: I78fdb16482f70714a8a8b64a9552e8874d7966fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282336
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed