Set duration for all video frames.

Set duration for all video frames.

Generally the muxer will ignore these, but there's one specific case
where it cannot: Since we flush before keyframes, the mov (MP4) mux
will not be able to set a proper duration based on the next frame
(usually it just does next_dts - dts), and thus guesses that it will be
the same as the previous one. If we dropped a frame between those two,
the duration of said last frame will be wrong -- and the keyframe
(starting in the next fragment) will get the wrong pts, possibly
seemingly even going backwards.

If we lose a frame between the last frame and the keyframe, the pts
of the keyframe will still be a bit wonky, but according to wbs
(who wrote the mux), it's much better like so.