Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

src/image/SkSurface_Gpu.cpp

Index: src/image/SkSurface_Gpu.cpp
===================================================================
--- src/image/SkSurface_Gpu.cpp	(revision 8710)
+++ src/image/SkSurface_Gpu.cpp	(working copy)
@@ -23,7 +23,7 @@
     virtual SkImage* onNewImageSnapshot() SK_OVERRIDE;
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
                         const SkPaint*) SK_OVERRIDE;
-    virtual void onCopyOnWrite() SK_OVERRIDE;
+    virtual void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
 
 private:
     SkGpuDevice* fDevice;
@@ -86,9 +86,8 @@
 // Create a new SkGpuDevice and, if necessary, copy the contents of the old
 // device into it. Note that this flushes the SkGpuDevice but
 // doesn't force an OpenGL flush.
-void SkSurface_Gpu::onCopyOnWrite() {
+void SkSurface_Gpu::onCopyOnWrite(ContentChangeMode mode) {
     GrRenderTarget* rt = (GrRenderTarget*) fDevice->accessRenderTarget();
-
     // are we sharing our render target with the image?
     SkASSERT(NULL != this->getCachedImage());
     if (rt->asTexture() == SkTextureImageGetTexture(this->getCachedImage())) {
@@ -96,8 +95,10 @@
             fDevice->createCompatibleDevice(fDevice->config(), fDevice->width(),
             fDevice->height(), fDevice->isOpaque()));
         SkAutoTUnref<SkGpuDevice> aurd(newDevice);
-        fDevice->context()->copyTexture(rt->asTexture(),
-            (GrRenderTarget*)newDevice->accessRenderTarget());
+        if (kRetain_ContentChangeMode == mode) {
+            fDevice->context()->copyTexture(rt->asTexture(),
+                reinterpret_cast<GrRenderTarget*>(newDevice->accessRenderTarget()));
+        }
         SkASSERT(NULL != this->getCachedCanvas());
         SkASSERT(this->getCachedCanvas()->getDevice() == fDevice);
         this->getCachedCanvas()->setDevice(newDevice);