public void flipFaceCull() {
        switch (cullMode) {
            case Back:  cullMode = FaceCullMode.Front; break;
            case Front: cullMode = FaceCullMode.Back;  break;
        }
    }
Flips the given face cull mode so that {@code Back} becomes
{@code Front} and {@code Front} becomes {@code Back}.
<p>{@code FrontAndBack} and {@code Off} are unaffected. This is important
for flipping the cull mode when normal vectors are found to be backward.
jme3-core/src/main/java/com/jme3/material/RenderState.java