Blender Git Loki

Avoid creating multiple outputs connected to the same socket when creating a node group

This patch fixes the annoyance that when creating a node group where one of its nodes is connected
to several other nodes, a separate output will be created for each link, even though they're all
connected to the same socket in the group.
Now, before adding an output for an outgoing link, the existing outputs are checked to find whether
any output is already connected to the same socket. If such an output is found, it is reused instead of
creating a new one.

Reviewers: Severin

Subscribers: Blendify

Differential Revision: https://developer.blender.org/D1836