This fixes the problem that matcaps have when using a very larg FOV in
perspective view.
This was because it was stupidly using the normal direction which can not
aligned with the view vector under perspective.
So to workaround this problem we don't use the normal as is, but compute
how much it's facing the camera.
This changes how matcaps looks in perspective because they now always use
the full range of the matcap (which is expected).
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