[graphite] Combine writeTextures and writeUniforms

RenderStep::writeUniforms and RenderStep::writeTextures act on the
same data structure in the same locations in code so it makes sense
to combine them into one method.

Bug: skia:13118
Change-Id: I48b2f5cc95247aa5d58c0780ead65378bb812ff5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/560396
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
16 files changed