Currently only supports one Mockingboard in slot 4, but it should be
fairly trivial to add another in slot 5. Tested with Ultima 3, 4 & 5,
and Mockingboard disk #1. Also, added some fixes to correct the timing
of the 6502 and sound; I believe I have a good understanding of it now,
even though there's still work to do to keep the main CPU thread from
starving the audio thread (which still happens, but less often now).
There was one bug that went undetected in the v65C02 core (in STX ZP,Y)
and another in the 80STORE handling (80STORE doesn't affect reading).
Also added a pause mode (bound to the Pause key), for those times when
you need to walk away from the emulation for a bit.
Before it was doing some complex timestamp thing, now it simple runs the
CPU for ~21 cycles and then stuff the sample into the sound buffer.
Sound still lags behind by several frames though, just like with the
previous sound setup. Still not sure why. :-/