video-player: set_playing: Don't do anything when not changing state
video-player: set_playing: Don't do anything when not changing state

This protects from doing any work with consecutive calls with the same
playing state.

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