TODO:
+ ability to run
+ ability to jump

sprites_to_draw:
+ jumping
+ swimming
+ getting out of pool
+ beast

pool:
+ only have beast on rock if it's not out

cavern:
+ spread slugs out a bit more
+ slugs on ceiling

underwater:
+ add arrival-underwater scene
+ ability to swim

beast:
+ add beast cutscene
+ add beast sprite

rope_room:
+ ability to grab rope in rope room

far-out:
+ add the shooting-in-hallway scene
+ ability to reach 1st checkpoint
+ implement intro movie