destroyGame() should not call exit()

[crankycyclops]

Right now, core is functioning as a library that an application (like standalone) wraps around. Since this is the case, core should not call exit() unless there's an unrecoverable error (like memory allocation failure), which means that destroyGame() should not call exit(). Should alter the main game loop in standalone/main.c so that it says something like:

while (inGame()) {
...
}

And then implement inGame() so that it returns TRUE if the game is running and FALSE if the game has ended. Then, initGame() will set the variable this function returns to TRUE and destroyGame(), when we're done tearing down, will set this value to FALSE.

DO EET!

[crankycyclops]

After while(inGame()) is done executing, the application can do whatever it needs to do before exiting.