Some fixes to `utils::WriteShapesPovray`:
Some fixes to `utils::WriteShapesPovray`:

- use ChSharedPtr::DynamicCastTo() in order to eliminate double casting (and C-style casting)
- ensure that we only count visual assets that are supported (and written to the output file)

hsu/chrono-vehicle,hsu/chrono-vehicle,hsu/chrono-vehicle