Add getter for channels to GrBackendFormat

Bug: skia:10078

Change-Id: I0172a87d836a606a55e60b73ba255a41313e9c12
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282266
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
10 files changed