When printing a character, or starting a new sixel image, purge all images that cover the cursor's current row.
This implements the CSI escapes for retrieving and (re)setting the palette size.
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
