intense flicker with sixel

[dankamongmen]

This kinda goes back to #1391, though we've improved quite a bit since then. Our sixel implementation results in a lot of flicker; this is especially noticeable with ncplayer -bpixel using video, but also in e.g. the yield demo. This just doesn't happen with mplayer -vo sixel (it admittedly does drop frames, but at any play rate, we see flicker, so that's not it).

Figure out exactly what's different between our use of sixel and libsixel's. I don't think this can be blamed on color quantization -- it definitely can't, because low-color videos have the same problem.

This needs be resolved before we can release 2.3.4.

[dankamongmen]

we don't see this flicker in our dankamongmen/libsixel branch, even in fullscreen mode. so either it comes down to libsixel's better perf, or something it's doing with its emissions that we are not. but we can absolutely resolve this -- it is within our powers.

[dankamongmen]

Hrmmm. This seems to be due to sprite_sixel_annihilate(). If it's disabled, we don't get this flicker, or at least tremendously less of it.

[dankamongmen]

The work going on in #1457 appears to eliminate all this flicker!!!