Nodes: Remove "Use Nodes" in Shader Editor for World

Part of https://projects.blender.org/blender/blender/pulls/141278

Blend files compatibility:
If a World exists and "Use Nodes" is disabled, we add new nodes to the
existing node tree (or create one if it doesn't) that emulates the
behavior of a world without a node tree. This ensures backward and
forward compatibility.

Python API compatibility:
- `world.use_nodes` was removed from Python API => **Breaking change**
- `world.color` is still being used by Workbench, so it stays there,
although it has no effect anymore when using Cycles or EEVEE.

Python API changes:
Creating a World using `bpy.data.worlds.new()` now creates a World with
 an empty (embedded) node tree. This was necessary to enable Python
scripts to add nodes without having to create a node tree (which is
currently not possible, because World node trees are embedded).

Pull Request: https://projects.blender.org/blender/blender/pulls/142342

intern/cycles/blender/addon/operators.py

@@ -12,20 +12,17 @@ from bpy.app.translations import pgettext_tip as tip_
 
 
 class CYCLES_OT_use_shading_nodes(Operator):
-    """Enable nodes on a material, world or light"""
+    """Enable nodes on a material or light"""
     bl_idname = "cycles.use_shading_nodes"
     bl_label = "Use Nodes"
 
     @classmethod
     def poll(cls, context):
-        return (getattr(context, "material", False) or getattr(context, "world", False) or
-                getattr(context, "light", False))
+        return (getattr(context, "material", False) or getattr(context, "light", False))
 
     def execute(self, context):
         if context.material:
             context.material.use_nodes = True
-        elif context.world:
-            context.world.use_nodes = True
         elif context.light:
             context.light.use_nodes = True
 

intern/cycles/blender/addon/ui.py

@@ -1456,7 +1456,7 @@ class CYCLES_OBJECT_PT_visibility_culling(CyclesButtonsPanel, Panel):
 def panel_node_draw(layout, id_data, output_type, input_name):
     from bpy_extras.node_utils import find_node_input
 
-    if not id_data.use_nodes:
+    if not isinstance(id_data, bpy.types.World) and not id_data.use_nodes:
         layout.operator("cycles.use_shading_nodes", icon='NODETREE')
         return False
 

intern/cycles/blender/shader.cpp

@@ -1639,9 +1639,7 @@ void BlenderSync::sync_world(BL::Depsgraph &b_depsgraph, BL::SpaceView3D &b_v3d,
     unique_ptr<ShaderGraph> graph = make_unique<ShaderGraph>();
 
     /* create nodes */
-    if (new_viewport_parameters.use_scene_world && b_world && b_world.use_nodes() &&
-        b_world.node_tree())
-    {
+    if (new_viewport_parameters.use_scene_world && b_world && b_world.node_tree()) {
       BL::ShaderNodeTree b_ntree(b_world.node_tree());
 
       add_nodes(scene, b_engine, b_data, b_scene, graph.get(), b_ntree);