Add GrBackendTexture/RenderTarget accessors to SkSurface

Change-Id: I63477fd4b8d48dc50af72736f0f8df566cd96d4a
Reviewed-on: https://skia-review.googlesource.com/85220
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
14 files changed