Add GrMtlRenderPipeline.

Creates a wrapper class just for id<MTLRenderPipelineState> so it can
be managed as a command buffer object.

Bug: skia:12253
Change-Id: Ibef833fda25d00e7b3110c17b59c5e760b523473
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/431040
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index ef82d75..d318b7c 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -784,6 +784,7 @@
   "$_src/gpu/mtl/GrMtlGpu.mm",
   "$_src/gpu/mtl/GrMtlOpsRenderPass.h",
   "$_src/gpu/mtl/GrMtlOpsRenderPass.mm",
+  "$_src/gpu/mtl/GrMtlPipeline.h",
   "$_src/gpu/mtl/GrMtlPipelineState.h",
   "$_src/gpu/mtl/GrMtlPipelineState.mm",
   "$_src/gpu/mtl/GrMtlPipelineStateBuilder.h",