qtdemux: do not use sparse streams in push-based seeking

qtdemux: do not use sparse streams in push-based seeking

Using the sparse streams can make the push-based seeking return
too far in the stream. It also can lead to issues as the
sparse streams will be ignored when restarting playback and,
 if the sparse stream is the one that has the earliest sample,
it will confuse qtdemux's offsets as one stream will have
an earlier offset than the demuxer's one which might lead to
early EOS.

https://bugzilla.gnome.org/show_bug.cgi?id=742661