support entity hierarchies in `CollModelManImplT::GetCM(filename)`
If a collision model is loaded from a
cmap file that keeps brushes not in the topmost Map (worldspawn) entity, but (also or only) in sub-entities, the resulting collision model is empty.
We should move all map primitives from sub-entities into the worldspawn entity, similar to code in
CaBSP/LoadWorld.cpp that moves map primitives of "static" entities into the worldspawn entity.
Alternatively or additionally, warn if
cmap collision models meet the above criteria.