playlist: let playlist-next go to first item if player is idle

Similar to the previous commit but the other way around. If you start
mpv as idle, load up a playlist without immediately hitting play and
then try to go to the next item, nothing happens. Naturally, you would
expect this to go to the first item. Fix this detecting if the playlist
has not started yet, the direction, and going to the first item in the
list.

common/playlist.c

@@ -207,6 +207,8 @@ struct playlist_entry *playlist_get_next(struct playlist *pl, int direction)
     assert(direction == -1 || direction == +1);
     if (!pl->current && pl->playlist_completed && direction < 0) {
         return playlist_entry_from_index(pl, pl->num_entries - 1);
+    } else if (!pl->current && !pl->playlist_started && direction > 0) {
+        return playlist_entry_from_index(pl, 0);
     } else if (!pl->current) {
         return NULL;
     }

common/playlist.h

@@ -71,6 +71,7 @@ struct playlist {
     struct playlist_entry *current;
     bool current_was_replaced;
     bool playlist_completed;
+    bool playlist_started;
 
     uint64_t id_alloc;
 };

player/loadfile.c

@@ -1753,6 +1753,7 @@ static void play_current_file(struct MPContext *mpctx)
     mpctx->playback_initialized = true;
     mpctx->playing->playlist_prev_attempt = false;
     mpctx->playlist->playlist_completed = false;
+    mpctx->playlist->playlist_started = true;
     mp_notify(mpctx, MPV_EVENT_FILE_LOADED, NULL);
     update_screensaver_state(mpctx);
     clear_playlist_paths(mpctx);