Environment map intensity

[mkeblx]

During discussion of environment map rotation (#117) the ability to control the environment map intensity was brought up. This would be also useful for: tweaking lighting, animating, and possibly handling LDR environment maps.

modelElement.environmentMapIntensity = ; // float, default = 1.0

References: Three.js: https://threejs.org/docs/index.html?q=scene#api/en/scenes/Scene.environmentIntensity