Blender Git Loki

Fix #36266, First undo in compositor decrements usercounter without a reason. The handling of scene backpointers in render layer/composite nodes changed slightly recently, which caused a double increment
of the scene user count. The node->id pointer for these nodes is now initialized in the respective init callbacks already. The explicit assignment and increment in the ED_node_composit_default is not
necessary and just adds an increment without checking previous values and decrementing them properly.

Note that the current system still leaves the scene with "fake" users (rlayer + composite nodes) which are actually part of the scene data itself. But that's design issue with the "local" node tree data
and doesn't do any real harm.