Issue with showing multiple volume renderings

Note, this issue is occurring in the GPU volume rendering mode, not the experimental multivolume mode

1. Load two volumes and volume render them both
2. Launch LookingGlass
3. Only the first volume viewable is visible in LG, while both are visible in Slicer

This can be worked around by launching LG before loading volumes

Reproduce the issue in GPU volume rendering mode by loading two volumes, volume-rendering both, and launching LookingGlass afterward; compare the LookingGlass view with the Slicer view. The fix is complete when both volumes are visible in LookingGlass without needing to launch it before loading the volumes.

Written by the indexing model from the issue text.