[graphite] Add buffer to texture copy support

Allows copying of multiple regions into a single texture, which supports
both filling mipLevels and rectangles in atlases.

Bug: skia:12845
Change-Id: I5fef9bda01125958a5491b4c5c8b3a66184357d2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/504056
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed