Sixel support; faster compressed graphical image transfers.

This release adds sixel support, it can be selected with --pixelation=sixel or, short -ps.
The sixel support has been tested with xterm (invoke with option -ti vt340), mlterm, xfce4-terminal (compiled with latest vte that allows for sixels) and alacritty-sixel.

Typically, you'd need to provide the -ps option if you want to test sixel output as auto-detection is currently only supported for xterm and mlterm.

Other than that, some few refinements here and there, faster compression for --pixelation=kitty and --pixelation=iterm2.