Revise GrMtlGpu::onUpdateBackendTexture.

* Use main command buffer rather than a temporary
* Use staging buffer for uploads
* Set up finishedCallback

Bug: skia:10530
Change-Id: I73dbf1466ec4a48de8c1dcb741fa1bb6de1cd554
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307230
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed