Increase specificity of GrColorType computed for YUV planes

When a single channel texture is used for a YUV channel we will
interpret it as kGray_8. When a single channel texture is used
as an A channel we will interpret it as kAlpha_8.

Change-Id: I746f8d761d7779266a4106311a93d651266bb422
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/232017
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
13 files changed