Add GrCompressedDimensions helper function

Bug: skia:9745
Change-Id: I75b5ba5767d354346694886adbfc55aaac8b0107
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/262231
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed