preliminary SkSL Metal backend

Bug: skia:
Change-Id: I03e839fb4f1061bc6d1b1b72d54fd39ca7bd7b84
Reviewed-on: https://skia-review.googlesource.com/51245
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
10 files changed