Fixed math and removed debugging artifacts.

The variable `gHackWindow' was part of some debugging I had done,
it doesn't belong in the code. Also, fixed an issue where screen
y-coordinates werre being computed upside down. Finally, improved
the grass tileset so that things look fancier now =].
Made the demo just a touch more `full'.

Lol! Added fullscreen. Also you can press `q' to quit, instead of
it just being a timeout.