Graphical fixes for DHIRES and added CASIN emulation.

Total Replay exposed (yet again!) a couple of flaws in apple2's
emulation: it would crash on the Serpentine demo and would display bad
graphics on DHIRES screen transitions.  The latter was caused by a lack
of CASIN emulation (and it's still not confirmed that it's correct), and
the latter was caused by bad handling of the the STORE80 softswitch.

Has it been almost a year since the last commit?  How time does fly...
:-/