added TextureRenderTarget to Metal gpu backend

Bug: skia:
Change-Id: I5cacdb832deefe0f8d8460ff10b2216d5dec1ed7
Reviewed-on: https://skia-review.googlesource.com/137890
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
6 files changed