Fixes #293

Reflectance and illuminant spectral distributions are different and must be treated differently when converting to sRGB. The difference essentially stems from that emitted radiance with equal energy for all wavelengths is not considered to be white, while surfaces that reflects radiance equally for all wavelengths are.

This PR uses the existing distinction between input illuminant and reflectance spectrums and applies the D65 white point to the integrated reflectance XYZ values before converting to sRGB.

I don't know of a scene where this makes much of a difference, because spectral reflectance distributions are only used for eta and k in metals as far as I'm aware. The difference seems to be close to eliminated in the Fresnel function for conductives when both eta and k are modified in the same way:

The color becomes a bit less greenish and more yellowish in places with lots of consecutive gold reflections, but it's barely noticeable. Gold is the only metal I've been able to see a slight difference with.

This is not the case if we specify k=0 however, or if we for example specify the diffuse reflectance for a material as a gray reflectance distribution:

Material "matte"
    "spectrum Kd" [ 300.0 0.5 
                    800.0 0.5 ]

Measured diffuse reflectance distributions exists and are useful for the spectral renderer, and this change should make sure that the result is the same when switching to RGBSpectrum.

Edit:

Another option for a better approximation is to use the Bradford E to D65 chromatic adaption:

static RGBSpectrum FromSampled(const Float *lambda, const Float *v, int n,
                               SpectrumType type = SpectrumType::Illuminant) {
    ...
    if (type == SpectrumType::Reflectance) {
        Float xyz_d65[3];
        xyz_d65[0] =  0.953188f * xyz[0] - 0.026590f * xyz[1] + 0.023873f * xyz[2];
        xyz_d65[1] = -0.038246f * xyz[0] + 1.028841f * xyz[1] + 0.009406f * xyz[2];
        xyz_d65[2] =  0.002607f * xyz[0] - 0.003033f * xyz[1] + 1.089253f * xyz[2];
        return FromXYZ(xyz_d65);
    }
    return FromXYZ(xyz);
}

At that point it's probably best to just include the D65 data and use it to compute the proper reflectance integral though.