When generating mipmaps, the non-base levels are redefined to be
compatible.  mRedefinedLevels was not updated to take this into account,
resulting in invalid copies to the image.

Additionally, noted a few spots where ImageDesc is used to respecify the
image, but those are not up-to-date when the backend functions are
called.  Changed those to directly get the necessary information from
the allocated image.

Bug: chromium:1094644
Bug: chromium:1094599
Change-Id: I2afc9e5a53f24ef56836c5d7eec2e3e11df0ef61
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2267423Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>