Changed Game::exit() to only perform a full shutdown when either FORCE_CLEAN_SHUTDOWN or GAMEPLAY_MEM_LEAK_DETECTION are defined. By default now (when these are not defined), it simply calls cstdlib ::exit(0), which causes immediate termination of the application. This was done to avoid extremely long shutdown times when trying to do cleanly delete every last pointer in the system, especially for large games. Any modern OS handles reclamation of memory, so there should be no adverse side effects.