Cleaned up level catalog file.
Improved renderEntity() to render entities more cleanly (no more pixel
jitter).
Defined data defs for on-road obstacles.
Added special object properties for object definitions so that specific
objects can be customized with properties.
Added a special property for on-road obstacle objects that defines the
obstacle hardness.
Added logic to racetrack system to process on-road obstacles.
Obstacles can have a hardness that determines whether to fully stop the
player or only slowing him down.