Improve opacity detection

If the opaque area covers all the window's shape, it means that it's an
opaque window.
This is how GTK presents opacity right now, so we'll get to skip
rendering under GTK clients and other compatible ones.
Qt is still not emitting opacity.

scene.cpp

@@ -294,6 +294,10 @@ void Scene::paintSimpleScreen(int orig_mask, const QRegion &region)
                 const QRegion shape = windowPixmap->mapToGlobal(windowPixmap->shape());
                 const QRegion opaque = windowPixmap->mapToGlobal(windowPixmap->opaque());
                 data.clip = shape & opaque;
+
+                if (opaque == shape) {
+                    data.mask = orig_mask | PAINT_WINDOW_OPAQUE;
+                }
             }
         } else {
             data.clip = QRegion();