Add CPU backing store for GrAtlas to reduce texture uploads

src/gpu/GrAtlas.h

Index: src/gpu/GrAtlas.h
diff --git a/src/gpu/GrAtlas.h b/src/gpu/GrAtlas.h
index d7f533414143214dce46d41c3bbf7cca4c52bdfc..b80de028882db2ee67e7c85adac04cf3f78a8e58 100644
--- a/src/gpu/GrAtlas.h
+++ b/src/gpu/GrAtlas.h
@@ -40,6 +40,8 @@ public:
     GrDrawTarget::DrawToken drawToken() const { return fDrawToken; }
     void setDrawToken(GrDrawTarget::DrawToken draw) { fDrawToken = draw; }
 
+    void uploadToTexture();
+
     void resetRects();
 
 private:
@@ -50,11 +52,14 @@ private:
     // for recycling
     GrDrawTarget::DrawToken fDrawToken;
 
+    unsigned char*          fPlotData;
     GrTexture*              fTexture;
     GrRectanizer*           fRects;
     GrAtlasMgr*             fAtlasMgr;
     GrIPoint16              fOffset;        // the offset of the plot in the backing texture
     size_t                  fBytesPerPixel;
+    SkIRect                 fDirtyRect;
+    bool                    fDirty;
 
     friend class GrAtlasMgr;
 };
@@ -82,6 +87,8 @@ public:
         return fTexture;
     }
 
+    void uploadPlotsToTexture();
+
 private:
     void moveToHead(GrPlot* plot);