theoraenc/oggmux: advertise fixed-framerate-ness properly, allow non-fixed framerate in theoranc for RTP etc.

Submitted by Tim Müller @tpm

Link to original bug (#753517)

Description

+++ This bug was initially created as a clone of Bug 748877 +++

Cloning this as a reminder and to collect ideas. Not sure if we really need to do anything or not.

theoraenc currently doesn't work with framerate=0/1 but enforces a fixed framerate. The fixed framerate-ness is really a property of the ogg mapping though, not of the Theora codec or library per se (fact?). If that is actually the case, we should add the framerate limitation for theora to the oggmux template caps and expand the theoraenc caps, so that theoraenc can produce/handle framerate=0/1 for RTP/other containers.