Selection highlighting in interactive session

[Vochsel]

To allow the selection of imageable primitives in an interactive session, aov's must be properly implemented.

Requires #14 to be implemented first

[Vochsel]

This seems to be impossible to support with vanilla cycles and "preview" renders. It's possible that once we get tiled rendering fixed for IPR we can do this, however that is currently not possible. Tiled rendering in cycles assumes at the end of the render you won't be updating the scene anymore.