qtdemux: respect qt segments in push-mode for empty starts
qtdemux: respect qt segments in push-mode for empty starts

In push-mode it is hard to support qt segments overall but it is
possible to support when the file isn't heavily edited but just contain
a segment to indicate a gap at the beginning. This also allows properly
timestamping data that has negative DTS in push-mode.

It is relevant to support those for 2 scenarios:

1) fragmented streaming
2) HTTP playback of 'regular' mp4

https://bugzilla.gnome.org/show_bug.cgi?id=753484

pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good