Allow swapping out of GrBackendTexture used by SkSurface

Change-Id: I67f26efaee87536f3faf51ca164bf003013c7c8a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215428
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
10 files changed