Add emissive color to PhongMaterial

PhongMaterial is lacking a "self-illumination color" (also called emissive color). It's the Color counterpart of the self-illumination image, just like diffuse color + diffuse image and specular color + specular image. This means that the lighting formula will be changed to:

 color = (ambient + diffuse) * diffuseColor * diffuseMap
             + specular * specularColor * specularMap
             + selfIlluminationMap + emissiveColor

This change has virtually no performance hit. The default value will have no effect (Black or null) to preserve backwards compatibility of the behavior.