I'm planning on supporting Text Mode soon, and SMS' VDP, when mode
4 is not active, behaves mostly like a regular TMS9918.
By having this behavior in a separate unit, we'll be able to use it
in other systems.
Add _TRA!, _THA!, _TRB!, _THB! routines to easily handle those pins'
value without stepping on other pins like the drivers previously
did. For SDC driver, it's going to be important soon because it turns
out that I can't get away with "always on" CS, so I'll need a scheme
where it's important that TH/TR pins have stable values.
Theoretically, it works. I can access an emulated SD card on it.
Will it work on real hardware?
I've also made SMS emulation faster. It was unbearably slow for SDC
access.
My idea of plugging a RC2014 bridge directly onto a Sega Master System
cartridge doesn't work. The SMS eats all I/O addr space, we can't use
it. Therefore, this naive idea, in the emulator, of reusing sdc.c in
sms.c as-is, doesn't work either.
I'll have to find another way of communicating to a SPI device on the
SMS. I'll probably do it through a controller port. Meanwhile, I need
to decouple SPI from SDC in the emulator code so that I can reuse
sdc.c. This is what is done here.
