libvlc: Make sure the media_player has its own state cached.

libvlc: Make sure the media_player has its own state cached.

A weirdness was introduced somehow. We were relying on the media state. (!)

Also, make sure that we always require stop() to be called when the media_player is destroyed. This may prevent some unwanted bad usage, whose consequence might be that _release() will block on input thread destruction.