Scene shader: Improve and document SH diffuse evaluation for light probes

[BlueCube3310]

Improves the spherical harmonics evaluation code based on https://github.com/kayru/Probulator/blob/master/Source/Probulator/SphericalHarmonics.h and https://cseweb.ucsd.edu/~ravir/papers/envmap/envmap.pdf .

As a result, the coefficients in the diffuse evaluation process more closely resemble the ones from the lightmapper, making the code slightly easier to comprehend.
Additionally, the magic numbers resulting from the convolution of directional SH are documented as equations.

Resulting numbers comparison:

SH coeff	Old	Old (calculated)	New	New (calculated)
0	0.886227	0.886227	0.886227	0.886227
1	0.511664 * 2.0 * y	1.023328 * y	1.023327 * y	1.023327 * y
2	0.511664 * 2.0 * z	1.023328 * z	1.023327 * z	1.023327 * z
3	0.511664 * 2.0 * x	1.023328 * x	1.023327 * x	1.023327 * x
4	0.429043 * 2.0 * x*y	0.858086 * x*y	0.858086 * x*y	0.858086 * x*y
5	0.429043 * 2.0 * y*z	0.858086 * y*z	0.858086 * y*z	0.858086 * y*z
6	0.743125 * z*z - 0.247708	0.743125 * z*z - 0.247708	0.247708 * (3.0 * z*z - 1.0)	0.743124 * z*z - 0.247708
7	0.429043 * 2.0 * x*z	0.858086 * x*z	0.858086 * x*z	0.858086 * x*z
8	0.429043 * (xx - yy)	0.429043 * (xx - yy)	0.429043 * (xx - yy)	0.429043 * (xx - yy)

[Repiteo]

Thanks!