Add support for USD material interfaces

[LouRohanNV]

• use GetValueProducingAttributes() for improved input resolution.
• use std::filesystem to avoid leaking file handles

In this zip file there are 3 flattened (no layer structure) examples:

• materials_no_interface.usda: no material interface, the Shader inputs contain the values (file, color, roughness, etc.)
• materials_interface.usda: material interface, the Shader inputs are connected to inputs in the Material prim
• materials_interface_instance.usda: material interface with instanced materials, the Shader inputs are connected to inputs in the Material prim and the Materials are instanced (this is a little more odd in these usda files because they've been flattened for this case)

materials_interface.zip

[havess]

Nice! Super useful thanks.