ShowPicture() and Stop() were racing, and the calling order from
neutrino seems to be not necessarily "correct". With gcc-4.8 /
glibc-2.16 binaries it happened that on switching to radio mode,
ShowPicture() was called and then the Stop() from stopping video
playback happened. This broke the stillpicture. Fix by adding more
locks and explicitly bailing out of Stop() if a picture is shown.