Add Phong shading to lobe visualizer, consider intensity-based colormap

[n8xm]

The current lobe visualizer assigns a color based on normal direction. We should move to Phong shading. Also, we could make it look cool by using an intensity-based colormap, where longer directions have a different color. (This is what BRDFLab does, see screenshot below).

[natevm]

I know at least in the vis world that rainbow color maps are actually looked down on, mostly because as humans, we're terrible at distinguishing subtle differences in hue. Colorblindness can also make rainbow maps difficult to read.

We might want to move over to a better colorscale, like a hot to cold scale, grayscale, etc.