When the caller issues a glCompressedTexImage call, we need to delete
the cached texture data, since the texture size/data are being
respecified and the old data is now stale.

This fixes MEC for "Klondike Adventures".

Bug: angleproject:5549
Change-Id: Ie788e2bf39f7a29ec6bc55f95d6f570f1d22d659
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2633071
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>