qtdemux: Use sidx to seek on fragmented mp4
At the moment, qtdemux completely refuses to seek on fragmented mp4 when operating in push mode.
This makes sense if the only source of fragmented mp4 is DASH or so, because the adaptive demuxer can do the seeking upstream. However, if we're just playing a fragmented mp4 via vanilla mp4, qtdemux needs to handle the seeking.
If there's an sidx present in the file, we can use that to find the correct moof and do seeking that way. Right now, qtdemux only uses the sidx to calculate duration and then discards it.