remove unused class
version 1.1

* game states: tutorial -> playing -> game over
* can now restart after game over
* restart from wave 3 if you've reached that wave
* fix lag when hitting first enemy
* key B toggles hitbox display
* ships must be 70% visible on screen before they can be hit
  (no more insta-killing them at the right edge)
add tutorial messages
fix boss collision and shot aiming
add boss, finish game
add readme; rename to main.rkt
add waves and dripper
chalk on blackboard
add chalk star tex
explosion effect when player is damaged
add explosion texture
spawners can detect when their enemies die
add enemy shots. game is fun now.
add enemy-shot-sheet tex
lower player ship layer
add clunker, health gauge, background