Merge MTLRenderPassDescriptors when possible.

Bug: skia:8243
Change-Id: Ib7ec0b66f62d64c495a8aa62d051e5d62c14b405
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210162
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
6 files changed