sixel support

[vprimachenko]

first and foremost thanks for the awesome work here!

i am very interested in adding sixel support, @Aetf do you have any pointers where to start?

[Aetf]

I briefly go over the wiki page of sixel, and it seems that the terminal needs to render those differently than a character.

Currently, the terminal state machine is implemented in https://github.com/Aetf/libtsm, and the screen as a big array of cells. So the initial step would be to modify libtsm to support store sixel info in cells. Then kmscon can be changed to render those cells accordingly.