Blender Git Loki

Fix T85966: Wrong link picked when dragging multi-input socket

The socket drag operator stored the index of the last picked socket
into RNA in case the mouse cursor leaves the link while dragging.

This id was not unique which is why sometimes a link from an other node
with the same id is picked.

This patch changes the way the last picked link is stored and stores a
pointer to the link directly into bNodeLinkDrag struct instead.

Differential Revision: https://developer.blender.org/D10590