[graphite] Add BackendTexture abstraction for Graphite.
There are definitely some missing getters on other functions we'll need
on BackendTexture. But for this CL I just wanted to get the basics and
ctor working. Other functions can be added as they are needed.
Bug: skia:12633
Change-Id: I668828138df943a432efcec6b1134756e681a582
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/469820
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
9 files changed