This allows fonts to be hot-swapped out by calling `set_font`
Unfortunately the way fonts are currently stored limits the maximum height to 8 pixels, since that's the size of a uint8_t and the y dimension is bitwise encoded.
The width can be arbitrary, by specifying the widths and max_width (the multiplier for number of bytes per char).
Might be worth grabbing some more bits from 32blit to make this not terrible, but now a user-defined font is an .hpp and a `set_font` away.
Phil Howard