Enable Blender's compositing viewer node in background mode

bg-viewer.patch

## SOURCE: https://blender.stackexchange.com/a/81240

diff --git a/source/blender/compositor/operations/COM_PreviewOperation.h b/source/blender/compositor/operations/COM_PreviewOperation.h
index 97d1884c9a7..1ccc570bac9 100644
--- a/source/blender/compositor/operations/COM_PreviewOperation.h
+++ b/source/blender/compositor/operations/COM_PreviewOperation.h
@@ -51,7 +51,7 @@ class PreviewOperation : public MultiThreadedOperation {
 
   bool is_output_operation(bool /*rendering*/) const override
   {
-    return !G.background;
+    return true;
   }
   void init_execution() override;
   void deinit_execution() override;
diff --git a/source/blender/compositor/operations/COM_ViewerOperation.h b/source/blender/compositor/operations/COM_ViewerOperation.h
index 7fc5ae36ad9..5ad1434fbd9 100644
--- a/source/blender/compositor/operations/COM_ViewerOperation.h
+++ b/source/blender/compositor/operations/COM_ViewerOperation.h
@@ -61,9 +61,11 @@ class ViewerOperation : public MultiThreadedOperation {
   void determine_canvas(const rcti &preferred_area, rcti &r_area) override;
   bool is_output_operation(bool /*rendering*/) const override
   {
+    /*
     if (G.background) {
       return false;
     }
+    */
     return is_active_viewer_output();
   }
   void set_image(Image *image)