version 0.98 pre: clipped sprites, more enemy-specific properties

+ random enemy is chosen from any number of enemies per level
* time to first enemy fire defined per enemy, not per level
+ CLIPPED sprites!! no more in/out popping enemies! wow...
* bullets/enemies removed when _entirely_ off screen
# enemies would sometimes be hit by bullets going right below them
# size of the second bullet was too big (invisible hit)
* the frequency an enemy fires bullets is defined per enemy
+ wide clipped sprites implemented (width 1-16 pixels)
# bosses first move left until x=100, otherwise they'd be off-screen