TODO:
+ slugs on ceiling
+ ability to run
+ ability to jump

+ have more slugs in cavern2 than cavern1
+ spread slugs out a bit more

+ add arrival-underwater scene

+ ability to change physicist_y in rope room
+ add beast cutscene
+ ability to grab rope in rope room
+ ability to reach 1st checkpoint

far-out:
+ add the shooting-in-hallway scene