For mutable textures, right now glTexImage call and some of other cases,
we are still using the ImageHelper's staging buffer. This will release
the staging buffer after flush to reduce memory footprint.

Bug: b/164511310
Change-Id: Ie1e52865a1c3a8f235c88331c4bb83c50d3f04da
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2378179Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>