Replace GrGpu::updateBackendTexture with narrower method that clears
a backend texture.
Creation of data for a solid color compressed texture is lifted up to
GrDirectContext and goes through updateCompressedBackendTexture.
Bug: skia:11786
Change-Id: I1d617623df5e65686f30e57c361a64f78d77f7bd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/392836
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
45889d1269