flvdemux: Handle seek flags properly
flvdemux: Handle seek flags properly

Allows for non-keyframe seeks.

https://bugzilla.gnome.org/show_bug.cgi?id=738570

Kurento/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,froggatt/gst-plugins-good-m,sebras/gst-plugins-good,StreamUtils/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,ikonst/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,froggatt/gst-plugins-good-m,rawoul/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,ikonst/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,chamois94/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,chamois94/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,rawoul/gst-plugins-good,froggatt/gst-plugins-good-m,hizukiayaka/gst-plugins-good,froggatt/gst-plugins-good-m