Wrap MTLRenderCommandEncoder in GrMtlCommandEncoderClass.

This will allow tracking to prevent setting duplicate state.

Change-Id: I7cc72125a5dce9efbd957f1dfffc85ce45681048
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/418146
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Adlai Holler <adlai@google.com>
12 files changed