Add GrMtlFrameBuffer.

Sets up creation of the framebuffer object from the current render
target and adds it to the OpsRenderPass.

Bug: skia:12186
Change-Id: I08984553bcef2afcf3003d3f59e40c2fa9e383a9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/430042
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed