Fixes#43733: "creating SpatialMaterial in a separate thread creates invalid
shaders (temporarily)."
The bug occurred because various setters called in materials' constructors add
materials to queues that are processed on the main thread. This means that
when the materials are created in another thread, they can be processed on the
main thread before the constructor has finished.
The fix adds a flag to affected materials that prevents them from being added
to the queue until their constructors have finished initialising all the
members.
