Blender Git Loki

Cycles X: Support shadow catcher behind transparent object

Seems to be rather straightforward.
Think initially it was less obvious to do, until we've started to count
number of samples for the shadow catcher pass.

Note that this is only about support of Transparent BSDF. Glass BSDF
can not be supported as it refracts light, which is not possible to
store as a shadow catcher pass.

Differential Revision: https://developer.blender.org/D11946