Fix last video skipping in Next mode

Reset _last_pos on play_file so a new video starting at 0
doesn't trigger the loop-restart detection from the previous
video's high position.

booru_viewer/gui/preview.py

@@ -441,6 +441,7 @@ class VideoPlayer(QWidget):
         self._current_file = path
         self._error_fired = False
         self._ended = False
+        self._last_pos = 0
         self._player.setLoops(QMediaPlayer.Loops.Infinite)
         self._player.setSource(QUrl.fromLocalFile(path))
         if self._autoplay: