vlm: store player finished state in media

This patch stores the finished state in the media context of VLM directly from the player callback.

The previous check on the player state to determine if the media was played was insufficient. The player state can go quickly from VLC_PLAYER_STATE_STOPPING to VLC_PLAYER_STATE_STOPPED and when it reaches the STOPPED state, there is basically no way to know if we are in the initial state or if we actually finished playing a media.

This was causing a deadlock where the VLM thread loop kept thinking the player was reset to its initial state and never switched to the next media.