[New Multicolumn] Correct paint rectangle calculation for compositing.

Source/core/rendering/RenderFlowThread.h

Index: Source/core/rendering/RenderFlowThread.h
diff --git a/Source/core/rendering/RenderFlowThread.h b/Source/core/rendering/RenderFlowThread.h
index effccc8e42f27e7346aa86085043af8868bad57f..2fe6e9bfce658800fc81d7cf7759ad2af75aa6cf 100644
--- a/Source/core/rendering/RenderFlowThread.h
+++ b/Source/core/rendering/RenderFlowThread.h
@@ -102,6 +102,11 @@ public:
     void collectLayerFragments(LayerFragments&, const LayoutRect& layerBoundingBox, const LayoutRect& dirtyRect);
     LayoutRect fragmentsBoundingBox(const LayoutRect& layerBoundingBox);
 
+    LayoutPoint flowThreadPointToVisualPoint(const LayoutPoint& flowThreadPoint) const
+    {
+        return flowThreadPoint + columnOffset(flowThreadPoint);
+    }
+
     void pushFlowThreadLayoutState(const RenderObject&);
     void popFlowThreadLayoutState();
     LayoutUnit offsetFromLogicalTopOfFirstRegion(const RenderBlock*) const;