Add GM configs that test rendering to a GL backend texture and render target

This also adds GrGpu::create/deleteTestingOnlyBackendRenderTarget. Implemented in GL only for now.

Change-Id: I9e5fdc953c4a249959af89e08332f520cefe9d90
Reviewed-on: https://skia-review.googlesource.com/113305
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
17 files changed