theoraenc: do not reset the encoder when we need a keyframe

theoraenc: do not reset the encoder when we need a keyframe

Instead, remember we need a keyframe, and we will force the encoder
to emit one next time we submit a new frame.
Since libtheora does not have an API to request a keyframe, we reset
the max keyframe interval to 1 temporarily.

This has the advantage that the rate control keeps its history,
and that the encoder won't choose different quant tables or
somesuch, thus requiring new streamheaders (although this is
probably only a theoretical possibility). Should also be a
bit faster than resetting the encoder.

https://bugzilla.gnome.org/show_bug.cgi?id=663350