add reminder notes to implement missing features in VIC-II sprites
note that active values of VIC-II sprite registers are in
vicii_sprites, not viciv.vhdl.
add some debugging for sprite drawing status.
fix VIC-II sprite X position calculation.
fix some VIC-II sprite drawing bugs
composite VIC-II sprites onto chargen image
fix bugs with drawing VIC-II sprites.
fix a VIC-II sprite rendering bug.
fix bug with drawing subsequent rows of sprites
fix some sprite drawing bugs