(MESS) nes_mmc5: * hacked around the limitations of our PPU code, so to support the 
different CHR registers
* improved PRG-RAM handling
* first attempt to add split screen mode, still broken
no whatsnew

the hack only deals with the way MMC-5 monitors PPU activity, not with the actual effect. when
I get to the point of completing my not-yet-abandoned PPU rewrite, the new calls in ppu2c0x.h
will go away...