Prevent the es_out from overriding the NowPlaying state by splitting
the meta into NowPlaying and ESNowPlaying.

The internal function input_item_GetNowPlayingFb can be used to return
the preferred meta value.

Stripped-down version of d4ccd7a5b404f62435c60ecc65f772dab83f5937
available on master.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>