Metal: Add attachment support for RenderTargets and TextureRenderTargets

Bug: skia:12186
Change-Id: I8004fd44e59c0dc3b9ae3bb5fc408421c4976e30
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/427756
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
10 files changed