playlist: add a input-current variable for being completely predictable

this new variable will allow interfaces to get a chance to add callbacks on a new input _after_ variables are created and _before_ the input thread is started.

In previous design, callbacks were added haphazardly while the input thread was already being executed.

src/playlist/engine.c

@@ -280,6 +280,7 @@ static void VariablesInit( playlist_t *p_playlist )
     var_Create( p_playlist, "playlist-item-append", VLC_VAR_ADDRESS );
 
     var_Create( p_playlist, "item-current", VLC_VAR_ADDRESS );
+    var_Create( p_playlist, "input-current", VLC_VAR_ADDRESS );
 
     var_Create( p_playlist, "activity", VLC_VAR_INTEGER );
     var_SetInteger( p_playlist, "activity", 0 );

src/playlist/thread.c

@@ -259,6 +259,8 @@ static int PlayItem( playlist_t *p_playlist, playlist_item_t *p_item )
         p_sys->p_input = p_input_thread;
         var_AddCallback( p_input_thread, "intf-event", InputEvent, p_playlist );
 
+        var_SetAddress( p_playlist, "input-current", p_input_thread );
+
         if( input_Start( p_sys->p_input ) )
         {
             vlc_object_release( p_input_thread );