ImageHelper::copyImageDataToBuffer() is called from glCopyTexture*. It
allocate staging buffer and write the copy command into command buffer
right away. This uses context staging buffer instead of ImageHelper's
staging buffer. This has the benefit of able to share staging buffer
with other objects (including buffers etc).

Bug: b/164511310
Change-Id: I3f680b1cd95df172a442aac573a8cc8d48972b1b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2364717
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>