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 .gpb
Merge pull request #1361 from staticfloat/encoderfix

Revamp light falloff calculation in Encoder