Non-manifold geom was producing inverted result when the camera was inside
the shadow volume.
When rendering non manifold geometry with the depth fail method, we need to
emit the cap as if it was the same geometry with a virtual thickness.
Another way to view it is to imagine having a set of non-manifold geometry
all facing the light.
So for every tri we emit a front cap oriented toward the light and the
back cap pointing away from it (whatever the actual orientation of the tri).
Extrusion pass stay the same as the depth pass method (double the increment
if it's a manifold edge).
