Metal: Use GrMtlFramebuffer than than GrRenderTarget where possible.

Bug: skia:12186
Change-Id: Ifb08a05619f856e1299ecddab0a88a0ed132dd57
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/430636
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed