MoviePlayer: don't track movies whose parents are also Movies
MoviePlayer: don't track movies whose parents are also Movies

(This could previously happen when a Movie swaps in a new child Movie on
a keyframe)
funkypandagame/flump,mathieuanthoine/flump,funkypandagame/flump,tconkling/flump,mathieuanthoine/flump,mathieuanthoine/flump,tconkling/flump