godot: Cannot draw in global space from a child node

Godot 2.0.3, Windows 10, 64 bits

I am trying to use _draw() to display a sort of particle system in global space, because the ParticleSystem2D is not suited for my case.

I don’t want to move my node to the root because I prefer to have it part of my objects, so I am using this workaround to counter all local transforms:

    var inv = get_global_transform().inverse()
    draw_set_transform(inv.get_origin(), inv.get_rotation(), inv.get_scale())

It appears to work, but if I scale the parent node the position is incorrect. Here is a minimal project showing the issue: GlobalSpace.zip

Is this a bug? Or am I doing something wrong?