Add GrMtlRenderTarget class

Adds basic support for creating non-msaa pure render targets.

Bug: skia:
Change-Id: I63d2d474b86fc0ff5d8ee7757c08abbfd5e6c6ef
Reviewed-on: https://skia-review.googlesource.com/31980
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed