This is a follow-on to
https://chromium-review.googlesource.com/c/angle/angle/+/2180881.

This adds a comment explaining part of that solution, and an end2end
test that creates a compressed texture and then renders after
decreasing MAX_LEVEL and then again after increasing MAX_LEVEL.

Bug: b/155499736
Change-Id: I04bb0dc9ead2807f7121e4c6b95ffd3594d5dcc0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2182174Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Ian Elliott <ianelliott@google.com>