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>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 6cea1d6..51ac125 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -586,6 +586,8 @@
   "$_src/gpu/mtl/GrMtlCaps.mm",
   "$_src/gpu/mtl/GrMtlGpu.h",
   "$_src/gpu/mtl/GrMtlGpu.mm",
+  "$_src/gpu/mtl/GrMtlRenderTarget.mm",
+  "$_src/gpu/mtl/GrMtlRenderTarget.h",
   "$_src/gpu/mtl/GrMtlTexture.mm",
   "$_src/gpu/mtl/GrMtlTexture.h",
   "$_src/gpu/mtl/GrMtlTrampoline.h",