qtdemux: guess bitrate if only one stream's bitrate is unknown

qtdemux: guess bitrate if only one stream's bitrate is unknown

If the bitrates for all but one audio/video streams are known, and the
total stream size and duration can be determined, this calculates the
unkown bitrate as (stream size / duration) - (sum of known bitrates).
While this is not guaranteed to be very accurate, it should be good
enough for most purposes.

For example, this is useful for H.263 + AAC streams where no 'btrt' atom
is available for the video portion.

https://bugzilla.gnome.org/show_bug.cgi?id=619548