implemented copy surface as blit for metal gpu backend

Bug: skia:
Change-Id: Ic59fe585c02168a361985f0864242b3c11e9d98e
Reviewed-on: https://skia-review.googlesource.com/142684
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
5 files changed