Add PolylineGlowMaterialProperty

This allows for DynamicScnee materials to make use of the PolylineGlow type.  Note, CZML does not support this material yet, but could be made to do so when we need to.

While this has been on the l low-priority list for a while, this thread specifically requested it: https://groups.google.com/d/msg/cesium-dev/CclK7a5cLmI/gCmd4Km4834J