Do we also want to write something to `material.diffuse`?  Glow adds intensity.  Compared to the outline material, this takes away intensity.  Yes, yes, thing will saturate at 1.0, I know, but I still think we can provide better artistic control.

In general, we are going to allow anything to glow as part of the post processing engine ([branch](https://github.com/AnalyticalGraphicsInc/cesium/tree/postprocess)), but when we can get something nice here, then this has the benefit of being much faster for polylines