[gltf] Implement KHR_lights options.

[donmccurdy]

Implementing remaining options in KHR_lights. I am not sure whether quadratic attenuation is supported in three.js, so it is ignored with a warning here.

References:

• Proposed schema (glTF 2.0: New KHR_lights extension KhronosGroup/glTF#945)
• Notes in Physically-based Distance Attention for Spot and Point Lights (backwards compatible) #5787, Physical quadratic falloff #6255, and Restored factor of PI to BRDFs #7523
• Valve: Constant-Linear-Quadratic Falloff
• Quadratic Lights in WebGL

/cc @takahirox

[donmccurdy]

Oh, I just found WebGLRenderer.physicallyCorrectLights, which appears to support quadratic attenuation? I'm not sure of the difference between quadraticAttenuation and fallOffExponent, and will look into that.

[mrdoob]

/cc @bhouston

[donmccurdy]

Ok, I think this is fixed. With quadraticAttenuation=2 and renderer.physicallyCorrectLights=true, glTF models should have physically correct lighting.

The fallOffExponent corresponds to our old spotlight.exponent parameter; I'm logging a warning on that for now and suggesting that the glTF2 KHR_light extension use a penumbra angle instead.

[mrdoob]

Thanks!