Adding the KHR_materials_iridescence effect (KhronosGroup#2027)

* Created KHR_materials_thinfilm branch.

* simplify thickness mapping by using `mix()`

* fix white space inconsistencies

* fix glsl code snippet highlighting

* add transmission specs

* add thickness spec

* add absorption spec

* remove optional dependency on thickness

* fix transmission spec

* Update README.md

* Update README.md

* refactor to refraction extension

* update thickness extension

* move thickness extension to its own branch

* initial draft

* improve thickness extension

* remove refraction related files as already present in KHR_materials_refraction

* remove absorption related files as already present in KHR_materials_absorption

* Minor fixes.

* Initial draft for EXT_lights_environment

* Removed thickness.

* Fixed some typos. Clarified the coordinate system.

* Added physical units.

* Removed nor required extension.

* Renamed everything.

* Added Ben.

* Updated iridescence schema to implement full model of Belcour/Barla

* Added line breaks.

* Removed the base extinction coefficient. Renamed the parameters.

* Added call of duty paper again.

* Added more references.

* Added reference.

* Add Appendix: Full Khronos Copyright Statement

* Add schema to readme

* Add overview

* Explain values

* Update overview

* Set state to draft

* Add reference

* Update header

* Sorted by company, then first name.

* Apply change request

* Update extensions/2.0/Khronos/KHR_materials_iridescence/schema/glTF.KHR_materials_iridescence.schema.json

Co-authored-by: abwood <[email protected]>

* Updated schema.

* Added another thin film link.

* Add implementation details and show effects of thickness and IOR

* Change default values of thickness min and max so that the effect is more colorful by default

* Clarify default texture value

* Add reference to Sample Viewer shader

* Iridescence: Update references

* Remove trailing whitespaces and update implementation details

* KHR_materials_iridescence: Add reference to the Schlick fresnel definition

* KHR_materials_iridescence: Set default value of thin-film IOR to 1.3 (water, alcohol, etc.)

* KHR_materials_iridescence: Update implementation details

* KHR_materials_iridescence: Put clearcoat extension interaction into normative section

* KHR_materials_iridescence: Make math formulas work in light and dark mode

* KHR_materials_iridescence: Make math formulas bigger

* KHR_materials_iridescence: fix line-breaks in Reference section

* KHR_materials_iridescence: Fix error in code example

* KHR_materials_iridescence: set thickness max. to 400 nm

* KHR_materials_iridescence: Update implementation details

* KHR_materials_iridescence: add missing closing parenthesis

* KHR_materials_iridescence: align links/references

* KHR_materials_iridescence: Use UTF-8 Pi instead of image

* KHR_materials_iridescence: Add padding around figures

* KHR_materials_iridescence: Use maximum RGB component instead of luminance for diffuse weighting

* KHR_materials_iridescence: remove clearcoat dependency

* remove unused images

* KHR_materials_iridescence: update default value of iridescenceIOR in schema

* KHR_materials_iridescence: Removed redundant parenthesis

* KHR_materials_iridescence: marked as Release Candidate

Co-authored-by: Ed Mackey <[email protected]>

* KHR_materials_iridescence: Add description for energy conservation approach

* Change iridescenceIOR to iridescenceIor

* KHR_materials_iridescence: Define behaviour when combined with KHR_materials_specular

* Update schema descriptions to match the core spec

* KHR_materials_iridescence: Fix issues with combination with KHR_materials_specular

* KHR_materials_iridescence: Fix parameter order in rgb_mix()

* Update to latest copyright statement.

Co-authored-by: Jim Eckerlein <[email protected]>
Co-authored-by: Jim Eckerlein <[email protected]>
Co-authored-by: Schoen, Pascal <[email protected]>
Co-authored-by: Mathias Kanzler <[email protected]>
Co-authored-by: UX3D-kanzler <[email protected]>
Co-authored-by: abwood <[email protected]>
Co-authored-by: Pascal Schön <[email protected]>
Co-authored-by: Ed Mackey <[email protected]>
Co-authored-by: Alexey Knyazev <[email protected]>