(MESS) nes_mmc5: misc wip [Fabio Priuli]
* hacked around the limitations of our PPU code, so to support the different CHR registers
* implemented Extended Attribute Mode (Ex1)
* improved PRG-RAM handling
* simplified NT-RAM handling
* first attempt to add split screen mode, still broken

out of whatsnew: this was for sake of messnew inclusion ;)