[mythtvnz] mythburn.py stream order

[Brett]

> I suggest that if this is of general application you should post it to trac
> 
Can't do that at present.

A basic requirement for the script is that it should always find the
video stream. That is general.
Not sure if this problem shows up anywhere else, have not found any
mention on mythtv/ffmpeg websites so could be New Zealand problem.

Have read comments on the mythtv mailing list that myth archive is due a
major rework.