dynbrick: be sure a brick exists.
dynbrick: be sure a brick exists.

Higher layers (e.g. the renderer) should really only know about
the bricks we've told it about.  Be sure it's not inventing
bricks.

git-svn-id: 353be9ac11a1e711854a13efcc2fc17d9d6b83e7@2114 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok