Redraw only dirty area for accelerated 2D Canvas.

Source/platform/graphics/GraphicsLayer.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 948 matching lines @@
 void GraphicsLayer::setIsRootForIsolatedGroup(bool isolated)
 {
     if (m_isRootForIsolatedGroup == isolated)
         return;
     m_isRootForIsolatedGroup = isolated;
     platformLayer()->setIsRootForIsolatedGroup(isolated);
 }
 
 void GraphicsLayer::setContentsNeedsDisplay()
 {
+    setContentsNeedsDisplayInRect(m_contentsRect);
+}
+
+void GraphicsLayer::setContentsNeedsDisplayInRect(const FloatRect& rect)
+{
     if (WebLayer* contentsLayer = contentsLayerIfRegistered()) {
-        contentsLayer->invalidate();
-        addRepaintRect(m_contentsRect);
+        contentsLayer->invalidateRect(rect);
+        addRepaintRect(rect);
     }
 }
 
 void GraphicsLayer::setNeedsDisplay()
 {
     if (drawsContent()) {
         m_layer->layer()->invalidate();
         addRepaintRect(FloatRect(FloatPoint(), m_size));
         for (size_t i = 0; i < m_linkHighlights.size(); ++i)
             m_linkHighlights[i]->invalidate();
@@ skipping 247 matching lines @@
 #ifndef NDEBUG
 void showGraphicsLayerTree(const WebCore::GraphicsLayer* layer)
 {
     if (!layer)
         return;
 
     String output = layer->layerTreeAsText(WebCore::LayerTreeIncludesDebugInfo);
     fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif