Add graphics modes

[jonathanpallant]

Need to implement some graphics modes and see what fits in the CPU time we have available.

We could use RLE to get things to fit, but I don't like that, as it will be even worse in the pathological case (black/white checker board). I'd rather it always worked in the worst case.

Also need to think about the horizontal-2x modes. Halve the clock speed of the PIO? Load a new PIO program to do 2x the delays? Load each pixel twice (probably not good from a CPU load and memory bandwidth point of view, given 320x200 wants to use the palette).

[thejpster]

Added 1-bpp mode in #88, which is possibly the most difficult mode?