quick backup of the finished scene loader before I submit other parts

changes

*Note: I made a change to simplify the recursive create_object function.
Since most nodes contain only one mesh, there's no point to iterate
through meshes, we just choose the first one available.

Also: despite the fact that our 3d model is flat (no node hierarchy) we
calculate its modelling transformation recursively (as a product of its
local transform with the accumulative transformation of its parent
nodes), this way we can safely replace the models anytime.