csgRebuild builds dynamic brushes from the whole world, instead of just the current level

Licensee notes issue with csgRebuild and proposes a reasonable fix.

1. Create a level with a couple of sublevels
2. Add a volume into each sublevel
3. Ensure current level is Persistent level
4. Rebuild geometry for currently level
5. Note that volumes are built even though they're not in the current level