Blender Git Loki

Cycles: Refactor shadow catcher ray visibility

Allow to "filter" shadow catcher objects by any of the visibility
(diffuse, glossy, and so on) to allow complete separation of objects
in the scene.

Implemented as a bitmask shift magic: shadow catcher objects have their
visibility "duplicated" as a shift left by 16. The ray visibility for
tracing will use either low or high visibility bits depending on whether
shadow catcher object was hit or not.

Currently no functional changes, just committing separately to make it
easier to bisect possible unintended functional changes.

Initial idea implementation from Brecht, adopted to the shadow catcher
code by self.

Differential Revision: https://developer.blender.org/D11158