Ffmpeg with x11grab

This allows to record a video of running selenium test on a test node. To record the video:

1. add --devtools option to firefox wrapper script ( to see jsconsole )

   vim $(find ~ -path */bin/firefox)

2. run script to catch x11 session and write video

   cd ~/srv/runner/public/

   ~/srv/runner/project/slapos/software/seleniumrunner/x11_video_catcher.sh

After test node fulfills a test task the output*.webm files should be in the current folder.

examples of video: https://[2001:67c:1254:e:c4::75e4]:9686/public/video8/

The version of the "seleniumrunner" should be changed to the containing patch if the video contains problems with fonts.

//cc @seb @romain