theoraenc: Don't reset the video quality setting the bitrate

theoraenc: Don't reset the video quality setting the bitrate

libtheora has two encoding modes, CBR, where it tries to hit a target
bitrate and VBR where it tries to achieve a target quality.

Internally if the target bitrate is set to anything other then 0 the
encoding-mode is CBR.

This means that the gstreamer element can leave the video_quality
setting alone as long as the user is tweaking the bitrate. Which has the
nice side-effect that if the user explicitely sets the bitrate to 0
(which is actually the default), the quality value doesn't get reset and
one ends up encoding VBR at quality-level 0...