added basic texturing to metal sksl backend

Bug: skia:
Change-Id: I2e70efe53f5c2fb208979cc89be9c3c841944a48
Reviewed-on: https://skia-review.googlesource.com/128844
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
2 files changed