Consider adding ability to generate a low resolution HDR cube map from SH

[kearwood]

Based on feedback from @elalish in the initial PR:

If we don't get permissions, can we still return a low-res version created from the output of XRLightProbe? That way we don't force anyone to implement spherical harmonics and directional lights if they just do IBL.

I would like to hear feedback from other UA vendors to see if they would also consider implementing a convenience function for generating low resolution HDR cube maps from the SH coefficients.

As there is computation involved and some renderers may be optimized for using SH coefficients directly, these HDR cube maps should not be calculated unless requested. Perhaps this could also be subscription based.