USDZExporter: Support opacity materials

[yuta-hayashi]

Fixed #22205
Related issue: google/model-viewer#2476

Description

Added support for displaying materials with transparent textures.

I implemented this by referring to the USD spec and RealityConverter's output.

Tested with Reality Converter on mac os and iOS 15.3.1 AR QuickLook, works!

The photos below were taken of this model using the AR QuickLook on the iPhone.

Before	After

[mrdoob]

That output is not correct, but it's pretty close.
I'll merge and experiment locally.

[mrdoob]

Thanks!

[mrdoob]

Actually, seems like the code is correct and their viewers are not behaving as the USD spec says:

@hybridherbst I found this related issue. Is this supposed to work now?

[hybridherbst]

I haven't tested usdview for quite a while (and not since they closed this issue). It had a lot of subtle bugs in rendering and there's no real reference viewer for USD, which makes deciding what's right harder than it should be...

My comment in that issue

they [QuickLook] seem closer to the spec but also not right

refers to exactly this incorrect behaviour of masking still becoming transparent, so the (wrong as per the spec) output in the photo above would be expected from a correct USDZ in the slightly incorrect QuickLook USD implementation. Never got a response to any of my bug reports to Apple unfortunately.