$Id$

  - Add ground and air vehicles, make it moon-buggy on steroids!

  - Provide several sets of sprites (monochrome, ANSI, UTF-8).

  - Let the ship pick up bonuses

  - Alien swarms, programmed behaviour

  - Spiral weapon (vertical sine)

  - Stick aliens to tunnel

  - Change tunnel colour

  - Draw bosses

  - Promote precision for all coordinates except screen

  - Animate the ship

  - The nuke should break the tunnel (or at least a bit)

  - The laser stays alive when the ship explodes

  - DONE 12 Nov 2003: Write a generic drawing library with automatic clipping
  - DONE 23 Dec 2002: Handle life
  - DONE 23 Dec 2002: Mega ball
  - DONE 23 Dec 2002: Fragmentation bomb (merge with mega-ball?)
  - DONE 23 Dec 2002: Fill holes in the tunnel when |step| > 2
  - DONE 19 Dec 2002: Tunnel sometimes doesn't get drawn on the left
  - DONE 18 Dec 2002: Draw a 3rd alien type