fmp4: Use updated start_pts when checking stream filled
After calculating the earliest pts, the fragment_start_pts and chunk_start_pts in State are updated. However, when checking if the stream is filled, the previous start_pts (set to None) is used instead. This means that chunk_filled and fragment_filled will be false the first time aggregate() is called, assuming timeout is false, all_eos is false, and the sinkpad is not EOS. This requires aggregate() having to be called a second time before the first fragment is sent.