Add support for using glCopyTexSubImage2D when possible to copy surfaces.

src/gpu/gl/GrGpuGL.h

Index: src/gpu/gl/GrGpuGL.h
===================================================================
--- src/gpu/gl/GrGpuGL.h	(revision 8655)
+++ src/gpu/gl/GrGpuGL.h	(working copy)
@@ -55,6 +55,8 @@
                                     size_t rowBytes) const SK_OVERRIDE;
     virtual bool fullReadPixelsIsFasterThanPartial() const SK_OVERRIDE;
 
+    virtual void initCopySurfaceDstDesc(const GrSurface* src, GrTextureDesc* desc) SK_OVERRIDE;
+
     virtual void abandonResources() SK_OVERRIDE;
 
     const GrGLCaps& glCaps() const { return *fGLContext.info().caps(); }
@@ -96,7 +98,6 @@
                                   const SkIRect& srcRect,
                                   const SkIPoint& dstPoint) SK_OVERRIDE;
 
-
 private:
     // GrGpu overrides
     virtual void onResetContext() SK_OVERRIDE;