This adds back sphere probe pre-convolution.
The difference is that we use spherical
Gaussian instead of the GGX NDF.
This allows us to reuse the previous mip as
a source for the convolution and thus reduce
the sample count and give a noiseless result.
However since we don't use filtered importance
sampling anymore, we have to compensate with
some more samples. This could be addressed in
a follow up PR if needed.
This also changes the octahedral mapping
procedure to avoid padding texels and
interpolation artifacts.
Also cleanup to make sure all functions
related to mapping are in the same file.
The change to Spherical Gaussian has some impact
on the look. The resulting visual is a less "foggy"
but most of the energy is where it should be.
Only the caracteristic "GGX tail" is missing.
These sphere light-probes convolved mips are only
used when raytracing is off or un-available (forward
surfaces).
Ref #118256
Pull Request: https://projects.blender.org/blender/blender/pulls/118354
b7998f1046