scene: Fix window pixmap traversal order

Since the last child window pixmap is the top-most one, it needs to be
pushed on the stack first. This may fix KMail rendering issues.

plugins/scenes/opengl/scene_opengl.cpp

@@ -1332,8 +1332,9 @@ void OpenGLWindow::initializeRenderContext(RenderContext &context, const WindowP
         contentRenderNode.leafType = ContentLeaf;
 
         const QVector<WindowPixmap *> children = windowPixmap->children();
-        for (WindowPixmap *child : children)
-            stack.push(child);
+        for (auto it = children.rbegin(); it != children.rend(); ++it) {
+            stack.push(*it);
+        }
     }
 
     // Note that cross-fading is currently working properly only on X11. In order to make it

scene.cpp

@@ -1097,7 +1097,10 @@ WindowQuadList Scene::Window::makeContentsQuads() const
 
         // Push the child window pixmaps onto the stack, remember we're visiting the pixmaps
         // in the depth-first search manner.
-        stack += windowPixmap->children();
+        const QVector<WindowPixmap *> children = windowPixmap->children();
+        for (auto it = children.rbegin(); it != children.rend(); ++it) {
+            stack.push(*it);
+        }
     }
 
     return quads;