These are done using a shadow mapping technique, we render both slides
from the point of view of the light, and then do a second pass in which
we lower the light of the fragment if some other cube is above it.

Change-Id: I8aaa1428c4481661283bf69b5e56aa4d95fb80dd
Reviewed-on: https://gerrit.libreoffice.org/22232Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>