Prior to this change, respecifying a texture image due to usage, base or
max level changes incurred a copy of every level and layer to a
temporary buffer which was then staged as an update to the new image.
This code was somewhat messy (for example with respect to depth/stencil
images), error prone (e.g. previously had bugs with compressed textures)
and disallowed further optimizations such as in anglebug.com/4835.

This change does the following:

- ImageHelper::SubresourceUpdate now takes ref-counted images, instead
  of image pointers.  This allows the same image to be staged for
  multiple updates.
- Respecifying an image is still done through a copy, but to an
  identical (temp) image instead of buffer, and each level of the image
  is staged as an update.
  * Further optimization is to stage the old image itself directly as
    updates to the new image

Bug: angleproject:4835
Change-Id: I4a3ef2d616c9ab459ff65f918b0fb6d9a2161b73
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2897537
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>