Delay half-float vertex color determination until Op::finalize

Bug: skia:8871
Change-Id: I873e80e2d97a32e4524c177a9b6e07b0566a02ce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201682
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
20 files changed