If the object is manifold and the camera is in the shadow side, we can
use the depth fail method to fix the inverted shadow glitch.
Unfortunately this does not really work for non-manifold.
Implementation details:
We try to be as efficient as we can, we precompute camera near plane
projected into 2D shadow space so we can test for intersection with the
shadow boundbox easily.
As the intersection test is done in 2D it's pretty fast.
Unfortunately, this means the shadow bounds are all aligned to the same
space and are not the smallest bound we could extract.
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