Revamp light falloff calculation in Encoder

* Remove Linear, Quadratic and Constant calculation from Light class
* Perform exact calculation of range rather than using approximation algorithm
* Remove falloffExponent as it was unused
* Default range parameter to 1.0f directly, rather than deferring until actually writing to the `.gpb` file

Please comment on these changes, I tried to clean up as much as I could, but I want to ensure that this doesn't break any functionality.  Additionally, I don't have Maya so I can't test the `DecayStart` code very well; Blender likes to encode things into `FarAttenuationEnd` as far as I can tell.