Differences compared to Mockingboard:
. TIMER1 underflows to $FFFE before it's reloaded with latch.
. TIMER1's LATCH=$0000 gets interpreted as $FFFF.
. Partial TIMER2 support: IFR.T2 gets set on underflow. But T2 reads as 0x00FF.
. AY register reads are unsupported (always read as 0).
Config->Sound: show 'MEGA Audio' in drop-down menus only if it was specified on the cmd line.
. For MB: isChipSelected[0] = true. So do this on Reset() or for Phasor when switching to MB mode.
. Add "Notes on Phasor's AY-3-8913 chip-select & r/w"
. Refactor some common code into SY6522::UpdatePortAForHiZ()
Mockingboard or Phasor cards can be inserted into any slot.
Change Config->Sound to that slots 4+5 to be individually selected for the 3 soundcard types.
Add MockingboardCardManager class to manage multiple cards and mix the sound buffers.
