Add testing-only backend render target support to GrVkGpu.

Update unit tests to use backend render targets on non-GL contexts

Add named DM configs for rendering to Vulkan backend render targets and textures.

Make src data ptr param to createTestingOnlyBackendTexture be const.

Change-Id: I17f5375ed9bb08422006698956469d3151c4954c
Reviewed-on: https://skia-review.googlesource.com/113276
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
12 files changed