Draft: Implement sixel support for displaying images with the GBM winsys.

This adds optional support for using libsixel to display test results when using the GBM window system framework. We used libcaca for this in the past, but libsixel produces dramatically higher image fidelity.

This can be really useful when testing drivers remotely over ssh.

Not all terminals support sixel yet, but many popular ones (Konsole, GNOME Terminal, alacritty, ...) have some support in their latest git branches.

TODO: detect terminal support for sixel and fallback gracefully to caca

For example, here are some comparisons between the output of glsl-1.50-geometry-end-primitive with libsixel and libcaca: