Switch to using GrMtlBackendContext for GrDirectContext creation.

Makes the Metal backend more consistent with the other backends,
and allows new init parameters to be added without significantly
changing API.

Added updated sk_cf_obj because I needed some of its functionality.

Bug: skia:10804
Change-Id: I6f1dd1c03ddc4c4b702ea75eff14bc0f98ab5ad2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334426
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
13 files changed