init multiple matter types (press C to switch which one is controlled)
basic fog of war
only add/remove changed tiles instead of regenerating every frame
add matter-keyed fog of war tiles
add pausing and rewinding
mark 'coherent' tiles
use selection image instead of transparency to indicate selected particle
allow giving commands in past\!
space instead of C changes selected matter type
ctrl/shift click to select multiple particles
update todo in notes.txt
don't let particles in formation move off map