Blender Git Loki

Nodes: Add function to set compact socket flag for vectors

This flag is currently only used for vector sockets
so the function is limited to the vector builder.

The flag is only used by two shader nodes at the moment
and this is needed to port them over to the new socket declaration API.

Reviewed By: JacquesLucke

Differential Revision: https://developer.blender.org/D13490