This uses context's staging buffer for immutable texture
TextureVk::setSubImageImpl call and flushes update right away.

Bug: b/164511310
Change-Id: I04fee0a9afe0e84617a461fb6cd7137e853adf8b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2357971
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>