demux: mp4: export ContainerChildren instead of ContainedRaw

So we can stop probing at some point.
demux: mp4: validate fragmentation after first moof
demux: mp4: fix seek return value
demux: mp4: add "Leaf" mode demuxer

Adds a data order driven demuxer instead of trying
to rearrange seek order. Re-enables fragmented
files if fast-seekable.