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kusma use full-screen fallback for all projection-plane intersecting spheres
Turns out, the previous code glitches in some more cases where the
occluder intersects the perspective plane. So let's use the
full-screen fallback in these cases.

While we're at it, remove that old, defined out version of the code
that is just as buggy and not in any way faster.
Latest commit 70b04db Oct 23, 2016