added global variable and multi-texture support to metal sksl backend

Bug: skia:
Change-Id: If676774ec0a30c5b536ccffbff2220d180b7fa59
Reviewed-on: https://skia-review.googlesource.com/129187
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
2 files changed