fix gltf/glb load for some models

This release implements a fix for loading of some gltf/glb files. The loading function now correctly uses the materials texture index's source image index to reference the images.

In general if you have issues loading gltf/glb files, try babylon.sandbox.com first. If it works there, please post a project with your model, so I can test it and debug.

Please note that with C3 SDK restrictions, only base color texture or solid color will impact rendering (e.g. metallic, normals, roughness, blender shaders will have no effect).