added interface block, vector comparison intrinsics, and mvk integration for metal sksl backend

Bug: skia:
Change-Id: Ia6984ba73482ec377f0890eb79f6ff92ff736749
Reviewed-on: https://skia-review.googlesource.com/129654
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
2 files changed