Long-term, we should figure out a better solution than hard-coding values like 0.4 here.  The problem is czm_phong uses "fake" light vectors in an attempt to ensure abstract objects are always lit for visibility, but, there are certain camera angles (particularly on translucent objects like sensor cones) where the diffuse light still falls to zero.  So here, we've mapped [0.0, 1.0] to [0.4, 1.0] to emulate ambient light, preventing the color from reaching full black.

Ideally, as @mramato suggested in #925, there would be a scene-level ambient light setting, that would apply the above `mix` to all phong lighting in the scene using a user-supplied value.