[glTF] Support double-sided materials

[footballhead]

All models loaded with scene::GltfLoader are assumed to be single-sided (back face culling is enabled). glTF supports double-sided materials. From 3.9.5:

When this value is true, back-face culling is disabled and double sided lighting is enabled. The back-face MUST have its normals reversed before the lighting equation is evaluated.

This would allow BigWheels to correctly render:

• TextureCoordinateTest