public void flip() {
        normal.negateLocal();
        flipped = !flipped;
        updateMesh();
    }
Flips this mesh by reversing its normal vector direction and
setting the {@code flipped} variable accordingly. This variable
will be used by the {@code updateMesh()} method to rearrange
the index buffer.
jme3-core/src/main/java/com/jme3/scene/shape/RectangleMesh.java