Getting Metal shader precompiling working.

* Adds GrMtlPipelineStateBuilder::PrecompileShaders.
* Adds GrMtlPrecompileShaderLibrary.
* Stores metadata with SkSL shaders so we can convert to MSL properly

Change-Id: Id2c23a54cc04ca3b61a639c1cbc7234b697c41ab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/376856
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed