Add GrCaps call to get swizzle from BackendFormat and ColorType.

This CL just adds the caps function. Follow on CL will use it thoughout
the gpu backend to remove GrPixelConfig dependency for swizzle.

Bug: skia:6718
Change-Id: Id299c72a9dc9f33411d8fd9c806158da20372474
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214188
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed