Export blender double-sided mesh flag to gltf materials.

[Cyp]

Just as making this pull request, it showed another new cb1f27b commit which does the same thing. So I suppose only either that one or this one will be useful, but not both.

Edit: Looking at that version, I think it wouldn't duplicate materials if needed, but this version should.

[emackey]

I tested this and it works really well, thanks! I do like that it will automatically duplicate materials when needed for differences in double-sidedness, that's great. I don't have any worries about letting go of that commit of mine you linked to.

I'll leave this open until @donmccurdy gets a peek at it, then we'll merge.

[emackey]

Fixes #154.

[emackey]

@donmccurdy The behavior resulting from this is:

• When "double-sided lighting" is enabled on a Blender mesh, the materials used by that mesh will be double-sided in glTF.
• When a double-sided mesh and a single-sided mesh share the same Blender material, the exporter will export two different copies of that material with the same name (allowed in glTF), wired correctly so the double-sided mesh gets the double-sided copy of the material, and the single-sided mesh gets the single-sided copy of the material.
• When meshes share materials and also share double-sided settings, they will continue to share glTF materials as before.
• Only single-sided materials can be influenced by the old custom nodes. But with this change, the custom nodes shouldn't be needed for this input anymore.

[emackey]

Merging this, thanks @Cyp.

[donmccurdy]

Nice clean fix, thanks! :)