Snapshot images appear as black frames

The ffmpeg command used to create snapshot images doesn't seem to work with all versions of FFmpeg. Currently it uses the option -itsoffset. This seems to work with FFmpeg 3:0.cvs20070307 from Medibuntu, but not with a more recent custom compiled version (SVN-r17187). With the latter the snapshots come out as all black images. Using another option, -ss, does work however. I haven't been able to test this option with the Medibuntu version, but it does work with an old version of ffmpeg bundled with FFmpegX (built in 2006). So -ss looks like a more compatible option.

This patch changes this option. I'm planning to commit this, if no one objects. If you can, please test the patch with your particular ffmpeg installation.