Use KHR_materials_transmission in FlightHelmet

[mrdoob]

Every engine seem to be handling alphaMode: BLEND differently (used in the glasses):

KHR_materials_transmission seems to have better alignment so I propose updating this model with the extension.

/cc @elalish

[andreasplesch]

On the other hand, the model shows off well what is possible without using extensions. An appropriate response may include addressing what the root cause of the differing interpretations is as the sample assets serve mostly this purpose.

[bghgary]

In Babylon.js, we turn on reflections by default because that's what most people expect. It can be turned off to be spec-conformant. We probably should enable this flag in the model viewer test code.

[elalish]

Yeah, the root cause of the differing interpretations is spec non-conformance because people wanted to do glass with alpha, which isn't correct, and hence why the transmission extension was added. I think this model should definitely be updated, because using alpha for glass is simply incorrect. @bghgary would you like to send a PR to update Babylon in our fidelity tests?

[elalish]

It appears I have the permissions, so I'm going to go ahead and merge this unless someone gives me a good reason not to. Is that okay @DRx3D?

[mrdoob]

Unable to fix these checks. Second attempt: #48