video-texture: Don't return in progress of 0.0 when seeking
video-texture: Don't return in progress of 0.0 when seeking

When seeking, the progress returned by playbin2 is 0.0. We want that to
be the last known position instead as returning 0.0 will have some ugly
effects, say on a progress bar getting updated from the progress tick.

lubosz/clutter-gst,skinkie/clutter-gst,ystreet/clutter-gst,ystreet/clutter-gst,lubosz/clutter-gst,GNOME/clutter-gst,ystreet/clutter-gst,GNOME/clutter-gst,skinkie/clutter-gst,ystreet/clutter-gst,lubosz/clutter-gst,GNOME/clutter-gst,GNOME/clutter-gst,skinkie/clutter-gst