need to finish:

	o  for large images, "pan" the viewport around:

		-xn	x coord of image mapped to (0,0) on screen
		-yn	y coord of image mapped to (0,0) on screen

it would be nice to do the following:

	o  read new map from file
	o  various convolutions and operations (e.g. histogram smoothing,
	   filters, etc)
	o  logically merge images (AND, OR, XOR, etc)
