GitLab

Use a box of six panels in lieu of a GL background color. The floor and
ceiling are now additional elements, drawn later, because they are
selecxtively enabled and disabled based on mode.

Other assorted cleanup along the way.

BUG=

Change-Id: Idbd5ad8188659bb5720ffa75c7e919ae5a2de469
Reviewed-on: https://chromium-review.googlesource.com/581850
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488799}