GLTFSceneModelLoader - backfaces option throw undefined if material in primitive.material is missing

[vsigno]

Describe the bug
In #1449 if GLTF material is undefined in line 523, const backfaces in line 537 throw an undefined TypeError when it tries to access doubleSided.

To Reproduce
It’s possible that the GLTF model I’m using has a missing material, however, it was loaded correctly using 2.6.0-beta13

Current fix
Setting param.backfaces explicitly to true when loading the GLTF model in GLTFLoader.load avoids the error. Even if true should be the default value for ctx.backfaces.

[xeolabs]

Fixed now in https://github.com/xeokit/xeokit-sdk/releases/tag/v2.6.2