demux: mp4: rework fragmented mp4
Extends the moov over http interleaves aware code to work over non seekable content (that's infinite preload until end of chunk). Replaces the experimental Leaf Demux which had some issues with data holes (Leaf did a reverted pos to sample time matching to fully avoid seeks). Does only lazy load chunks/runs and allows fragmented level sample seek (must have seek mfra index).
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