opengl: Don't cache the menubar background.

It can change based on the _toolbar_ size.  The toolbar itself is fine though;
it may depend on the menubar size, but that should be constant.

Change-Id: Id2c68c89f4ec0774bc74b8c4a5b9a5e59e22137f
Reviewed-on: https://gerrit.libreoffice.org/18279Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

include/vcl/salnativewidgets.hxx

@@ -287,6 +287,12 @@ public:
             case CTRL_CHECKBOX:
             case CTRL_RADIOBUTTON:
                 return false;
+
+            case CTRL_MENUBAR:
+                if (mnPart == PART_ENTIRE_CONTROL)
+                    return false;
+                break;
+
             default:
                 break;
         }