When transitioning between formats in cube maps, tracking of luma
workaround information would become incorrect because it was only tracked
per-level.

There are no failing tests due to this because once the cube map is
complete again, the tracking is correct.

TEST=conformance/more/conformance/quickCheckAPI-S_V.html

BUG=angleproject:2074

Change-Id: If4e1e94061208f0cf3668b2bd6be59a9a21e42b6
Reviewed-on: https://chromium-review.googlesource.com/539956
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>