This adds support by just reusing the GGX reflection LTC
look-up table. This avoid more memory usage for another
table.
This is quite a hack and has no real physical ground.
We already have a roughness remapping function for
reusing sphere-probe for refraction and matching the
blur level. We can reuse this function and use it
for sampling the reflection LUT.
Then getting the theta LUT parameter is done by
computing the angle between the refraction direction
and the reversed normal.
This works because the table is parametrized using the
angle between the view vector and the normal. This angle
is the same as the angle between the reflection vector
and the normal. So to get the equivalent lobe in the
refraction direction we get the angle between the
refraction direction and the reversed normal.
Note: This has issues shadow-map tagging but it should
be fixed separately.
Pull Request: https://projects.blender.org/blender/blender/pulls/118589
038fc84ab3