Deleted obsolete SWRacingScreen class.
Added support for essential RT objects. Entities of objects or object
collections marked as essential are placed in the game world evewn if
they otherwise would be left out depending on the entity thinning
multiplier. So for example setting the multiplier to 4 will only draw
every 4th entity but entities that are essential are drawn anyway. This
is useful for entities that should be mandatory like collectible items
and checkpoints.