KHR_materials_dispersion (#2340)

KHR_materials_dispersion (#2340)

* draft of EXT_materials_dispersion

* incorporating feedback

* use inline latex

* fix typo

* specify defaults and range in extension.

* make clear that it is an enhancement on top of KHR_materials_dispersion

* make clear the names used in other material models.

* add references from OpenPBR.

* change to storing 20/V

* improve equations

* wording cleanup.

* split equations apart.

* test

* improve the json to match dispersion spec

* address @lexaknyazev's feedback

* remove mention of texture

* Some updates based on review (GitHub and PBR TSG) feedback.

* Update visible light wording

Co-authored-by: Bastian Sdorra <bsdorra@gmail.com>
* Change $V$ to $V_d$ per review, add note about internal reflections

* Update to reference Cauchy's equation

Co-authored-by: Bastian Sdorra <bsdorra@gmail.com>
* Fix blank line

* Add note box per review.

This didn't appear in VSCode's markdown preview, but we'll test GitHub.

* Lambda fix per review

* Tweak

* Status changed from "Draft" to "RC" in an official PBR TSG vote today.

* Contributor list update

* Clarify Fraunhofer spectral lines and define lambda symbols

Co-authored-by: Bastian Sdorra <bsdorra@gmail.com>
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Co-authored-by: Ed Mackey <elm19087@gmail.com>
Co-authored-by: Bastian Sdorra <bsdorra@gmail.com>
Co-authored-by: Alexey Knyazev <3479527+lexaknyazev@users.noreply.github.com>