'Restores previous zoom level.'

def zoom_out(self):

    if (len(self._zoom_stack) < 1):
        new_translation = QVector3D((-0.5), (-0.5), (-0.5))
        new_scale = QVector3D(1, 1, 1)
    else:
        (new_scale, new_translation) = self._zoom_stack.pop()
    self._animate_new_scale_translation(new_scale, new_translation)
    self._zoomed_size = vec_div(QVector3D(1, 1, 1), new_scale)
