Move the ability to access textures, buffers, and image storages out from GrProcessor.

Move the ability to access textures, buffers, and image storages out from GrProcessor.

GrXferProcessor can no longer use this functionality so it is moved to a new intermediate class inherited by GrFragmentProcessor and GrPrimitiveProcessor.

Change-Id: I4f30c89bdceb2d77b602bf0646107e0780881c26
Reviewed-on: https://skia-review.googlesource.com/11202
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>