mode7 speed fixes:
	+ Don't draw sky every frame, only if needed
	+ faster multiply routine
	+ move multiply vars to zero page
	+ leave one of multiply results in accumulator at end?

	+  Skip key parsing if no key read

short term:

longer term:
	sky -- color looks too much like grass on monochrome display
	map -- adjust x/y to better match real position
	name entry screen
	inventory screen
	collision detection
	after game over, return to title screen
	save/restore game