[Cin] MKV containers

[Andrea paz]

Use 1080p and 30fps smartphone movies, or capture video (vokoscreen)
under the same conditions. Using the video driver as X11 (use direct
X11 render if possible) slightly improves the situation compared to
X11-openGL. But the real difference is that it disables "Play every
frame" (as Pierre has also noticed). All other settings in the
"Playback A*" tab have no effect at all. However, the situation gets
worse and worse, now slows down even without transitions or effects,
just go from one edit to the next to get a block and then resume
normally. In the meantime I made a backup of my system, because I see
it ugly for the future.