Fix for impossible reflections.
Is there a reason why this wouldn't work?
The basic idea is to check whether the normal is actually facing the light before performing the LTC lighting.
This check is only performed for specular reflections, since the artifact seems to only affect those.
Perhaps the check could be made more accurate, but in my tests it solved the artifacts and the demo files that I tried rendered identically.
(Except for the Mr. Elephant scene, where the paravent (?) in the background seems to show the artifact, which is of course resolved by this.)