Large scale shadows

From the forum: https://groups.google.com/d/msg/cesium-dev/25jPqr5ODg0/79dIxAjVDQAJ
(See also https://groups.google.com/forum/#!msg/cesium-dev/INkyXhFVcf4/iS6QyZckDwAJ)

Shadows are tuned to objects closer to earth, at a range of about 5000 meters, so larger scale views from space like this don't cast shadows:


Start by reading the two linked Cesium forum discussions and comparing their context with the provided screenshot. Identify how shadow behavior is tuned for approximately 5,000-meter ranges, then define and verify behavior for views from space where large-scale objects should cast shadows.

Written by the indexing model from the issue text.