Add support for fog in materials.

[ghost]

As per thread: https://hub.jmonkeyengine.org/t/add-fog-to-lighting-j3md/41873/6

I have added fog to multi-pass lighting.j3md/lighting.vert/lighting.frag for consideration.
I have not made any changes to SPLighting.vert/.frag or PBRLighting yet because the implementation would be the same, so once this has been approved, it can be ported to the rest of the materials.

I have also included a TestLightingFog in the jme-examples to test the three available fog-modes.

• Linear (simple, lightweight, useful on low-end and android devices)
• Exponential (medium - prettier but slightly more expensive than linear)
• ExponentialSquared (expensive compared to the previous two, but the best looking).

The user sets the parameter "UseFog" to enable fog, and must set a FogColor.

• setBoolean("UseFog", true);
• setColor("FogColor", new ColorRGBA(0.5f, 0.6f, 0.7f, 1.0f);

Setting one of the following settings will enable that fog type. This is how you decide which fog-type to use.

• setVector2("LinearFog", new Vector2f(near, far));
• setFloat("ExpFog", 0.015f);
• setFloat("ExpSqFog", 0.02ff);

All of this is described in code in the TestLightingFog class.
https://github.com/jayfella/jmonkeyengine/blob/master/jme3-examples/src/main/java/jme3test/light/TestLightingFog.java