Function roughnessToExponent

roughnessToExponent(roughness): number
Convert a roughness to specular exponent

Uses the conversion from - http://simonstechblog.blogspot.com/2011/12/microfacet-brdf.html
Parameters
- roughness: number
  The roughness to convert to shinyness/exponent
Returns number
The resultant shinyness.
- Defined in core/canonical_material.ts:38