RaP: Presentation: Don't process audio twice when no video.

The audio is unconditionally processed earlier in the script. Remove the
line that runs it again when there is no video in the recording.
RaP: Presentation: Support old recordings with '.pdf' in presentation name

In some old recordings (0.80?) I've seen the default presentation pdf
located at the filename .../default.pdf/default.pdf. Support this old
location with fallback logic so the new scripts will work to reprocess
old recordings.

I've improved the error handling logic here a bit as well.
RaP: Presentation: Fix panzoom generation to handle no slides

It's a pretty rare case, but I've seen some recorded meetings that had
no slides (not even the default pdf). Handle this without crashing.
Merge pull request #144 from kepstin/recording-fixups

Recording fixups