As a first implementation, this makes the copy go through the staging
buffer.  Once anglebug.com/4719 is implemented, this can be optimized to
perform the copy directly.

Bug: angleproject:2914
Bug: angleproject:4274
Change-Id: I2c9863381f54c3467ca939d049336960c21f60b0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2240671
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>