Move open questions to a Future work section (#113)

* Remove obsolete remarks.

* Emission future work.

* Bump maps as future work.

* Metadata as future work.

* GTR future work.

* Thin-film iridescence future work.

* Thin-wall future work.

* Metal Fresnel future work.

* Anisotropy future work.

* Subsurface future work.

* Transmission future work.

* Coat future work.

* Address review comments.

* Remove note on bump maps.

* Remove note on transmission_extra_roughness.

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Co-authored-by: Adrien Herubel <[email protected]>