This makes a lot of shader node wiring code less complex.
This only add the flatten mechanism (which fixes T69672).
~~Cleanup to remove the complexity in ntree_shader_relink_displacement,
ntree_shader_bump_branches and ntree_shader_tag_nodes will be commited
separately.~~(already included)
The code is only added for shader nodes for now but could be exported to
other internal nodetree types in the future.
Cleanup: Node Shader Tree: Remove GPUmaterial special nodegroup handling
Reviewers: brecht
Reviewed By: brecht
Maniphest Tasks: T69672
Differential Revision: https://developer.blender.org/D5829
3a1b0c0f30