Better USD suppport

Because blender's USD export capabilities are limited, we are stuggling to add blender to our pipepline.

Features:

• exporting glass materials correctly
• Having a proper scene structure organizer window or use collections as groups when exporting and maintains it when importing in another DCC like Houdini or Maya's Stage importer
• Referencing USD as Layers in Blender which "imports" the scene but others in the team can still work on the same USD file and update it. Which fullfills the USD defination that allows multiple departments to work on the same shot simultaneously using Layers.

Can stack layers: Layout.usd + Animation.usd + Lighting.usd

• Support "Payloads" (load heavy assets only when needed) and "Variants" (switch a car from Red to Blue).
• MaterialX support. There is already a checkbox in current USD export tab but it doesn't seems to work as intended.

Even imitating Houdini's USD workflow a little bit would help a lot with integrating Blender in professional multi-software pipeline.

I know I am asking for a lot. But If Blender adds anything similiar to the functions I mentioned above, this will play a much more critical role in it becoming a part of professional pipeline than any other flashy features. The industry is conviced that Blender is awesome, lets just help them setup an easier and already established pipeline. Thanks :)