AssImp loader blocks quick3d materials

1. Run the cube.qml example with qmlviewer with Qt/3D configured with "old_importer".
2. Result is good, with a green material on the cube (see cube-good.png)
3. Run it against with Qt/3D configured to use AssImp.
4. Green material has been replaced with the default grey material (see cube-bad.png).

The "Cube" element is implemented under "src/imports/shapes/Cube.qml", by importing a cube.obj file. The issue seems to be AssImp adding a default top-level material that overrides the intrinsic material set on the QGLPainter by the Effect.