LeafDemuxer fixes (the experimental data-sequence driven
fragmented mp4 demux).

Some muxers are interleaving using single trun/fragments,
Moov()->Moov(A)->Moof(B)
instead of interlacing truns withing fragments.
Moov()->Moof(A,B)->Moof(A,B)
Example: BigBuckBunny ismv file

Another problem is that data isn't interleaved at all
at end of tracks, and there's a long trailing gap of
audio fragments only.
Moov()->Moov(A)->Moof(B)->Moof(B)->Moof(B)->Moof(B)