Minor fix to procedural reflection map calculation.

[abwood]

Clamp horizonDotNadir calculation to effectively push any subsurface position up to the surface. Without this, geometry positioned close to the center of the Earth will turn white.

Note this does not target master. This change builds on @emackey's current pbr-lighting branch.

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[hpinkos]

Thanks @abwood! I'll let @emackey review, but you'll need to submit the same change to gltf-pipeline on the 2.0-cesium branch

[emackey]

Thanks @hpinkos, I'll take care of copying to gltf-pipline.

Looks like we need to add @abwood to the corporate CLA list for Concierge.

[emackey]

This is for #6430

[hpinkos]

@emackey yep, I added Alex to the CLA list