glTF and qgltf: Use the common materials extension

Right now ggltf provides its own shaders. This is not ideal since we would like to use Qt 3D's built-in materials as much as possible. This will become even more important with multiple lights because it is not necessarily realistic to expect that kind of support from qgltf's simple built-in shaders.

https://github.com/KhronosGroup/glTF/tree/master/extensions/Khronos/KHR_materials_common provides a way to define phong and similar materials without an actual shader. This is perfect for many cases. A minor issue is that the extension is a bit incomplete (there's more than just Blinn, Phong, Lambert and unlit, f.ex. normal mapping).