RaP: video processing robustness improvements

Makes it handle possibly corrupted video files with less chance of
breakage. Use features from newer ffmpeg release to simplify the video
trimming - in particular, using -ss as an input option is accurate now.
Switch recording processing to use poppler instead of ghostscript
Use the timestamp from the meeting ID to calculate the real start time

With the switch to a different clock source in red5, the timestamps
in the events file are no longer real-time, but are rather in
seconds since computer boot.

The timestamp in the meeting id is still realtime, so use that as
the start time. Calculate the end time by adding the meeting length
to the start time.