Can't create a ShadowMap object using public API

[slozier]

I'm trying to use a light source other than the sun so I figured I could just change the shadowMap of the scene. But to create a new ShadowMap it looks like I need to specify a Context which is not part of the public API.

[pjcozzi]

We weren't planning on exposing multiple shadow maps until Cesium support multiple light sources so this constructor should actually be private and not in the reference doc.

In the meantime, you can, of course, use Context and friends, but the API is may change.

[fredj]

See also #4001