Function position_texture 

pub fn position_texture(
    rd: &mut Gd<RenderingDevice>,
    sink: &Arc<dyn RidSink>,
    width: u32,
    height: u32,
) -> RdTexture

RGBA32F storage texture (compute writes world positions, material samples). One texel per vertex: the surface material reads VERTEX from here because a spatial shader cannot sample the verts storage buffer the readback uses.