TODO [GFX]: Fix darkened shadows in light probes

[steaklive]

When rendering to a light probe (i.e. specular), it is very noticeable that shadows are way too dark. Perhaps, a fake opacity value can be added to a probes-specific shader permutation. We don't need a physically accurate solution for probes, just something that won't ruin their convoluted radiance with pitch-black texels.