Fix T52021: Shadow catcher renders wrong when catcher object is behind transparent...

Fix T52021: Shadow catcher renders wrong when catcher object is behind transparent object

Tweaked the path radiance summing and alpha to accommodate for possible contribution of
light by transparent surface bounces happening prior to shadow catcher intersection.

This commit will change the way how shadow catcher results looks when was behind semi
transparent object, but the old result seemed to be fully wrong: there were big artifacts
when alpha-overing the result on some actual footage.