Don't use color type derived from GrPixelConfig to determine proxy texture swizzle.

Change-Id: Ib19e378d7dd8243cbbee68e97bcb3b6159943500
Bug: skia:6718
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233898
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
25 files changed