[graphite] add textureSize intrinsic

* exposes `textureSize` intrinsic across shaders, which was already present in compute shaders the form of `textureWidth` and `textureHeight`.

#ai-assisted

Change-Id: Ic6490e244715964150122149ba003b09bf85beac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1200976
Commit-Queue: Thomas Smith <thomsmit@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>