Planar graphics mode

[thejpster]

#37 talks about adding an addition multi-colour mode, but we're still limited to 4 colours per block.

To get a full 3-bpp image, it's most efficient to store it as 3x monochrome 1-bpp planes (red, green and blue). This would take 40.5 KiB of RAM at full 384x288 resolution, but a workable 20.25 KiB at 192x288 (or 384x144) or ~10 KiB at 192x144.

The 192 width modes need a u8 -> (u8, u8) expansion function, or to drop the clock rate to 10 MHz, re-adjusting all the timers.