Fix bug where a dedicated Toolstrip Renderer would not be called for background painting.

Author: KlausLoeffelmann

src/System.Windows.Forms/System/Windows/Forms/Controls/ToolStrips/ToolStrip.cs

@@ -3648,6 +3648,12 @@ protected override void OnPaintBackground(PaintEventArgs e)
                 }
             }
 
+            if (Renderer.RendererOverride is ToolStripRenderer renderer)
+            {
+                renderer.DrawToolStripBackground(new ToolStripRenderEventArgs(g, this));
+                return;
+            }
+
             Renderer.DrawToolStripBackground(new ToolStripRenderEventArgs(g, this));
         }
         finally