Save CPU power: generate only single thumbnail for each recording segment

[ondrej-zary]

For each recording segment in Grid view, 3 thumbnails need to be generated by running slow ffmpeg (and having it to seek to the middle which is a slow operation). This makes the Grid view unusable with slow CPUs. I suggest an option to generate only one thumbnail per recording segment - from the first frame (omitting ffmpeg -ss parameter), avoiding seek.

Executing ffmpeg is a slow operation by itself (around 0.9s on Atom D245 CPU). Further improvement could probably be done by using the ffmpeg-provided libraries instead of executing external program.