Scene-level ambient light setting

[emackey]

We use eye-based "fake" light vectors in czm_phong to keep abstract objects always lit. For example, sensor cones are not physical objects that you can see in the real world, so we want them to be visible under all lighting conditions in Cesium, without regard to placement of actual light sources.

This is a request to add a scene-level ambient light setting, to help keep abstract objects like this always lit. A sample implementation for sensor cones is hard-coded in #985, and further discussion is in #925.

[pjcozzi]

Handled for now in #1680. More lighting plans are on the roadmap.