Add GrTextureRenderTargetProxy and support switching between RT & Tex Proxies

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4427

Change-Id: Ie7662299953592f564bb27a4df4ea101f743403e
Reviewed-on: https://skia-review.googlesource.com/4427
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
11 files changed