[ganesh] Add buffer->buffer transfer task

Bug: skia:12720
Change-Id: Ic26b1fa35c8323b2be99993d207ce90be7456d02
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565897
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed