Fix the motion blur effect

It used to work by coincidence, as it used tr.orientation that was
being overwritten by AddEntitySurface. Also the shader didn't consider
the vector in the right space. Two points to keep in mind:
- MatrixTransformNormal2 is just a = m * a without the affine part (and
  not a normal transform)
- The Z component was in the wrong direction, I have no clue why. Sorry.