`ApplyShadow` result discarded during shader creation

[ajtribick]

The result of the ApplyShadow call here isn't used.

https://github.com/CelestiaProject/Celestia/blob/b54fa98b0d3a4b4f12887f42210363622f250dea/src/celengine/shadermanager.cpp#L2467-L2469

Need to check what this does and either append the code to source or remove the call.