viewport material export

[julienduroure]

New option to export simple material, based on viewport display information.

Exported values:

• BaseColor (RGBA)
• Metallic (float factor)
• Roughness (float factor)

=> This is very fast, as we don't check the node tree.
=> Custom properties of the material can still be exported

[julienduroure]

Note: We are close to 4.3 release, so we can't make this (new feature) into it.
This is planned for 4.4

[HailToDodongo]

Thanks for looking into this so quickly!

I did gave the PR a test on the latest 4.3 beta, and it does seem to fix my issue.
Export works again, and the basic material + custom properties are still there for me to process.

The only thing i noticed is that certain older models have wrong vertex colors, but i can create a separate issue for that.
(As a test, .obj export has correct colors)