Add GrGpu buffer transfer function, impl on Metal, add test

Also rename existing transfer buffer alignment cap for surface transfers to distinguish from new one for buffer to buffer transfers.

Bug: skia:13278
Bug: skia:12720
Change-Id: Idc4116c72fc2852a01d40feec6584d2bde9eae82
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/550517
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
14 files changed