If a mesh is known to be manifold, then it's not necessary to increment the
stencil buffer 2 times anymore. But we still need to account properly for
degenerate triangles.
In this case, only generate a quad if the tri is facing the lamp. If there
is a degenerate loop, the other edge will either cancel the increment (if
it is also facing the light) or not produce a quad (if not facing).
This will always give the correct count.
d3def53be5