Things to do

 * let the ship pick up bonuses

 * handle life

 * alien swarms, programmed behaviour

 * sprite library

 * mega ball

 * spiral weapon (vertical sine)

 * fragmentation bomb (merge with mega-ball?)

 * stick aliens to tunnel

 * change tunnel colour

 * fill holes in the tunnel when |step| > 2

 DONE Dec 18 2002: draw a 3rd alien type

 * draw bosses

 * port to conio.h

 * clip graphics 

 * promote precision for all coordinates except screen

 * tunnel sometimes doesn't get drawn on the left

 * write a generic drawing library with automatic clipping

 * write a linux console output for the drawing library

 * animate the ship