Blender Git Loki

Fix for [#24293] Shadow pass is wrong

* The problem is that shadow pass is derived from the diffuse pass as
shad = shad'/diff, where shad' = shad*diff. In cases where diff is
0 and the division can't be done shad is left as shad' (=0).
* This all works just fine until the diffuse color is 0 on just one
channel (no red in material color for example). In this case the shadow
pass is left as 0 too regardless of the existence of an actual shadow,
so the end result is a colored shadow!
* The only real solution is to use the original shadow intensity to
determine if there actually is a shadow or not. This is now stored in
shr->shad[3] from the lamp shadow calculation.

Note: The best solution would probably be to calculate the shadow pass on
it's own and not to derive it from the diffuse pass, but I didn't dare to
start messing up the shading code totally.