[glTF] Support double-sided materials

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](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#double-sided):

> 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](https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/TextureCoordinateTest/glTF-Embedded)