RGBI video support

[thejpster]

While we don't really have the memory or the SPI outputs to add an extra channel for true 16-colour RGBI output, there is a hack we could do.

We could apply the I bit on a per line basis. This would basically involve lifting a GPIO pin before the visible area and dropping it after the visible area. We would need a circuit where intense black was the same level as non-intense black (like on a ZX Spectrum), but we would then only need 1 bit per line, and one conditional GPIO write per line.

I need to research a circuit for doing RGBI TTL video.

[thejpster]

We'd need to turn the border off otherwise it'll be stripy.