demux: mp4: move fragmented duration computation
demux: mp4: move fragmented duration computation

tracks aren't init yet, so we can't compute duration
when falling back from missing mehd

vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc