Add GrMtlUtil class

Currently just adding support functions to go back and forth between
GrPixelConfigs and MTLPixelFormats.

Bug: skia:
Change-Id: I01a7d6877ebed87b87090ac2b920fee45dc0e856
Reviewed-on: https://skia-review.googlesource.com/29080
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed