Monsters chase after you, plus better debugging support

You can press the escape key to switch into debug mode dynamically now instead
of having to change the variable and reload.

Also, monsters chase after you now!
Mobs damage you (but you can't die yet)
Add the rock texture I've bee using.
Work towards making it an actual game.

Added a shape for the goal.

Added an 'invincibility' setting, accessible while the game's in debug mode.

There's supposed to be a transition to the "dead" state when your health hits
zero, but it's not working.