This flag is only used a few small cases, so instead
of setting the flag for every node only set the
required flag for the nodes that require it.
Mostly the flag is used to set `ntype.flag = NODE_PREVIEW`
For nodes that should have previews by default which
is only some compositor nodes and some texture nodes.
The frame node also sets the `NODE_BACKGROUND` flag.
All other nodes were setting a flag of 0 which has no purpose.
Reviewed By: JacquesLucke
Differential Revision: https://developer.blender.org/D13699
- Many cleanups of to use list base
- Some variable changes
These change is needed to migrate to the new socket builder API
Reviewed By: manzanilla
Differential Revision: https://developer.blender.org/D12366
