Produce: Drop frames with a PTS equal or lower to the last frame

In some cases, we end up with frames that have an FFmpeg PTS equal to
the previous frame. When this happens, both libx264 and FFmpeg will
start complaining and we get a bunch of logging about frame writes
failing. I suspect this happens because we have nanosecond scale PTS in
KWin/PipeWire but FFmpeg only has millisecond scale PTS, so we end up
with frames with the same millisecond PTS after narrowing. Since FFmpeg
refuses to write these frames anyway, we may as well drop them before we
even pass them to the encoder.