xfade: only emit images from the first tag list we get that has them

GStreamer now re-emits the whole tag list when something changes, so
we pointlessly re-decode the same jpeg several times with some files.
playbin: only emit images from the first tag list we get that has them

GStreamer now re-emits the whole tag list when something changes, so
we pointlessly re-decode the same jpeg several times with some files.