[graphite] Better encapsulate SkPipelineData's texture data

We are going to cache the uniform data and the texture data separately.
Begin by making the texture data something that could be cached on its own.

Bug: skia:12701
Change-Id: I1847f5c901939fcb445694e7cc65e903a6c7c822
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/522179
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed