Blender Git Loki

ShaderNodeTree: Add flatten function to get rid of nodegroups

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