Add option to GrGpuBuffer::updateData to *not* discard non-updated area.

Also support minimal map region in Dawn implementation when onUpdateBuffer uses mapping.

Bug: skia:13427
Change-Id: I5c8a2872b520e04f8a55085c86430e635da2f43f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/553585
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
22 files changed