Add Support for Missing Properties

[crazyjackel]

Good Day,

I'm currently working on a Rust rewrite of Assimp and have been using tobj to assist with OBJ file parsing.

While comparing how tobj and Assimp handle material definitions, I noticed that several properties commonly found in .mtl files are parsed by Assimp but not explicitly provided by tobj. These values are instead accessible via the unknown_param field.

To help improve compatibility and completeness, I’d like to provide an account of the following properties:

• Emissive Map (map_emissive, map_Ke)
• Normal Map (map_Kn, norm)
• Reflection Map (refl)
• Displacement Map (map_disp, disp)
• Roughness Map (map_Pr)
• Metallic Map (map_Pm)
• Sheen Map (map_Ps)
• Roughness Factor (Pr)
• Metallic Factor (Pm)
• Sheen Factor (Ps)
• Clearcoat Roughness Factor (Pcr)
• Clearcoat Thickness Factor (Pct)
• Anisotropy Factor (a)
• Transmission Color (Tf)
• Transmission Alpha (Tr)
• Emissive Factor (Ke)

Thank you for your hard work on this project.