only set color index if using dual source blending in Metal gpu backend

also implemented setting blend constants to allow non-dual source blending

Bug: skia:8230
Change-Id: Id342e525f3a0b6a3e676f81058661f5a31e66843
Reviewed-on: https://skia-review.googlesource.com/146762
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
5 files changed