Moves method to get GrBackendFormat from GrBackendTexture from GrCaps
to GrBackendTexture so that a GrContext is not required.
Uses kUnknown_GrPixelConfig as failure return from GrCaps functions
rather than an GrPixelConfig* out param and bool result.
Having the texture type be part of GrBackendFormat made removing the
GrCaps function that goes from GrBackendRenderTarget to GrPixelConfig
awkward so that was left alone for now.
Change-Id: If9be0f898c538be4a7b24022b6011f63441a0317
Reviewed-on: https://skia-review.googlesource.com/c/175991
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
f391d0f771