We were only tracking one image per level of cube map textures.
Instead, we need to track one per face (6).

Test: MEC for Extreme Car Driving Simulator
Bug: b/180419767
Bug: angleproject:5735
Change-Id: I59e6a5e83a60666a29f44d0a1e1993a1b461e8e7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2744293Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>