Consider removing `XRLightProbe.sphericalHarmonicsOrientation` and `XRReflectionProbe.orientation`

[kearwood]

The orientation of the platform specific API's lighting estimation may not match the XRSpace orientation. XRLightProbe.sphericalHarmonicsOrientation and XRReflectionProbe.orientation were included to avoid the cost of generating rotated versions of SH and cube maps.

Rather than storing the relative orientation, perhaps the UA should generate rotated variants aligned with the XRSpace of the viewer for global lighting estimation.

[kearwood]

If the UA generates cube maps, it should also generate mip maps. We should also define the cube maps to be in linear space, both for correct mip map generation and use within shaders.