[graphite] Add copyTextureToBuffer call on CommandBuffer.

Bug: skia:12466
Change-Id: I44bd3899dea817318f83444f023c1c8adcfc2d1e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/459881
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
6 files changed