Per- mateial specular boost for workbench shader

When working on complex high poly objects and designs there's seldom need for pbr complexity(ie eevee) when setting up material definitions.

-Currently there is no way to modify the specular intensity of the workbench shader on a per material basis (iirc it was removed a good few versions ago).

-Adjusting roughness will spread the highlight in a way that ruins the visual of the color underneath.

Having an individual boost control to the phong term in the workbench shader would solve these issues.