[graphite] MtlComputeCommandEncoder

Added MtlComputeCommandEncoder and a WorkgroupSize data structure to
represent global and local work group sizes.

Bug: b/240604572
Change-Id: I71e2af983411e6c02e6b326315b88084e9c1af40
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/563518
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed