Don't blend transparent pixels when rendering layers.

With this change, the rendere keeps track of what regions are rendered into
and generates a mesh that matches these regions exactly. The mesh is used
to composite the layer on screen.

Change-Id: I1f342576b9134fb29caff7fb8f4c1da179fe956d
diff --git a/libs/hwui/LayerRenderer.h b/libs/hwui/LayerRenderer.h
index ed5d960..f2fb898 100644
--- a/libs/hwui/LayerRenderer.h
+++ b/libs/hwui/LayerRenderer.h
@@ -49,12 +49,17 @@
     void prepare(bool opaque);
     void finish();
 
+    bool hasLayer();
+    Region* getRegion();
+
     static Layer* createLayer(uint32_t width, uint32_t height, bool isOpaque = false);
     static bool resizeLayer(Layer* layer, uint32_t width, uint32_t height);
     static void destroyLayer(Layer* layer);
     static void destroyLayerDeferred(Layer* layer);
 
 private:
+    void generateMesh();
+
     Layer* mLayer;
     GLuint mPreviousFbo;