A client can re-use the palette between images. Resetting the palette
breaks this.
Now we initialize the palette on demand, and resets it when the
palette size is changed (by the client).
This implements basic parsing of sixel data. Lots of limitations and
temporary solutions as this is still work-in-progress:
* Maximum image size hardcoded to 800x800
* No HLS color format support
* Image is always rendered at 0x0 in the terminal