Blender Git Loki

Geometry Nodes: Multi-Input Sockets

Normally sockets only have one input link. This commit adds the back-end
changes needed to use multiple input links per socket.

Multi-input sockets can be defined with a new flag in `bNodeSocketType`.
The changes necessary to make the sockets work in the geometry nodes
evaluator are generalizing input socket values as a vector of values,
and supporting this in the derived node tree structure.

This patch should contain no functional changes. Two upcoming patches
will use this system for the "Join Geometry" node and expose link picking
and updated display in the UI: D10069 and D10181.

Reviewed By: Jacques Lucke, Hans Goudey

Differential Revision: https://developer.blender.org/D10067