Nodes: implement operator node.activate_viewer()

The goal is to have a clear separation between active nodes and active viewers in a node tree. Both the compositor and geometry nodes viewers are supported. This will also allow us to implement shortcuts for viewers for geometry nodes.

A viewer node can be activated using
```
viewer_node = bpy.context.scene.node_tree.nodes["Viewer"]
with bpy.context.temp_override(node=viewer_node):
    bpy.ops.node.activate_viewer()
```

Pull Request: https://projects.blender.org/blender/blender/pulls/134456

source/blender/editors/space_node/node_edit.cc

@@ -1804,6 +1804,62 @@ void NODE_OT_preview_toggle(wmOperatorType *ot)
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
 }
 
+static int node_activate_viewer_exec(bContext *C, wmOperator * /*op*/)
+{
+  SpaceNode *snode = CTX_wm_space_node(C);
+  PointerRNA ptr = CTX_data_pointer_get(C, "node");
+  Main *bmain = CTX_data_main(C);
+  bNodeTree *ntree = nullptr;
+  bNode *node = nullptr;
+
+  if (ptr.data) {
+    node = static_cast<bNode *>(ptr.data);
+    ntree = reinterpret_cast<bNodeTree *>(ptr.owner_id);
+  }
+  else if (snode && snode->edittree) {
+    ntree = snode->edittree;
+    node = bke::node_get_active(*ntree);
+  }
+
+  if (!node) {
+    return OPERATOR_CANCELLED;
+  }
+
+  if (node->is_type("CompositorNodeViewer")) {
+    for (bNode *other_node : ntree->all_nodes()) {
+      if (other_node->type_legacy == node->type_legacy) {
+        other_node->flag &= ~NODE_DO_OUTPUT;
+      }
+      node->flag |= NODE_DO_OUTPUT;
+
+      WM_main_add_notifier(NC_NODE | NA_EDITED, &ntree->id);
+      WM_main_add_notifier(NC_SCENE | ND_NODES, &ntree->id);
+    }
+  }
+  else if (node->is_type("GeometryNodeViewer")) {
+    /* Geometry nodes viewers don't rely on NODE_DO_OUTPUT flag alone. */
+    viewer_path::activate_geometry_node(*bmain, *snode, *node);
+  }
+  else {
+    return OPERATOR_CANCELLED;
+  }
+
+  BKE_main_ensure_invariants(*bmain, snode->edittree->id);
+  return OPERATOR_FINISHED;
+}
+
+void NODE_OT_activate_viewer(wmOperatorType *ot)
+{
+  ot->name = "Activate Viewer Node";
+  ot->description = "Activate selected viewer node in compositor and geometry nodes";
+  ot->idname = "NODE_OT_activate_viewer";
+
+  ot->exec = node_activate_viewer_exec;
+  ot->poll = ED_operator_node_active;
+
+  ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
+}
+
 static int node_deactivate_viewer_exec(bContext *C, wmOperator * /*op*/)
 {
   SpaceNode &snode = *CTX_wm_space_node(C);

source/blender/editors/space_node/node_intern.hh

@@ -374,6 +374,7 @@ void NODE_OT_preview_toggle(wmOperatorType *ot);
 void NODE_OT_options_toggle(wmOperatorType *ot);
 void NODE_OT_node_copy_color(wmOperatorType *ot);
 void NODE_OT_deactivate_viewer(wmOperatorType *ot);
+void NODE_OT_activate_viewer(wmOperatorType *ot);
 
 void NODE_OT_read_viewlayers(wmOperatorType *ot);
 void NODE_OT_render_changed(wmOperatorType *ot);

source/blender/editors/space_node/node_ops.cc

@@ -44,6 +44,7 @@ void node_operatortypes()
   WM_operatortype_append(NODE_OT_hide_socket_toggle);
   WM_operatortype_append(NODE_OT_node_copy_color);
   WM_operatortype_append(NODE_OT_deactivate_viewer);
+  WM_operatortype_append(NODE_OT_activate_viewer);
 
   WM_operatortype_append(NODE_OT_duplicate);
   WM_operatortype_append(NODE_OT_delete);