stageSubresourceUpdateAndGetData call are made and then flush the
commands out, so they could just switch to use context's staging buffer
instead of use per object's staging buffer.

Bug: b/164511310
Change-Id: Iff7944a37073bb3641498e334847f599903858b1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2376895
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>