Emissive component for PhongMaterial

[Nir Lisker]

Hi Johan/Kevin,

Iv'e submitted an issue for adding an emissive component to PhongMaterial
[1].
This component is the single-color counterpart of the self-illumination
map. It functions the same way as diffuse color is to diffuse map and
specular color is to specular map.
Emissive colors are used to simulate an object giving off light since they
do not interact with external lighting.

Implementation will consist of simple changes to both 3D rendering
pipelines.

Please evaluate,
Nir

[1] https://bugs.openjdk.java.net/browse/JDK-8221720