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.

[mhagnumdw]

It would be really good to have an option to specify the number of thumbnails.

Another interesting option for low-end CPUs is to specify the number of threads needed to generate the thumbnails.

[matteius]

I seem to recall in the last few nights implementing something around this, would be worth having another check if I got it right in the new release.

[ondrej-zary]

Great, the thumbnails are working fine now!