KHR_lights_punctual: Support intensity

[Moguri]

Support intensity property for KHR_lights_punctual

[Moguri]

This can be multiplied into the color value. Blender 2.7x was using watts for power already and blendergltf was just multiplying this into the color value. In other words, for spot and point lights, if we convert the luminous intensity (candela) value back to power, we can then multiply that value with the color. This snippet found in this Blender glTF IO issue should help:

- Kv (683) - maximum spectral luminous efficacy of radiation for photoscopic vision,
  for 555 nm wavelength, that is most perceptually efficient for human vision
- Φe - radiant flux [watts]
- Φv - luminous flux [lumens]
- Iv - luminous intensity [lm/sr or candela], what KHR_punctual_lights mandates for point lights
--
Φv = Kv * Φe
Iv = Φv / 4PI
=>
Lv = Kv * Φe / 4PI // for point light source

In other words, we want to multiply color by Φe, which we get by:

Φe = Lv * Ω / KV
Where Ω is the solid angle of the shape:
  * 4PI for sphere
  * 4PI * sin^2(θ/2) for a cone with an angle of θ
Where Φe is 683

For directional lights and KHR_lights/Blender 2.7, we would multiply the Irradiance (watt/m2) against the color. I am not sure how accurate this is, but it seems like as good a place as any to start. KHR_lights_punctual gives us an intensity value for directional lights in lux (lm/m2) that we need to convert into watts/m2. To get watts from lumens (lm) is Φe = Φv / Kv, which I think should also work for getting watts/m2 from lux (i.e., using a conversion factor of 1/Kv).

Another note, this all assumes 100% luminous efficiency, which is probably okay.