Add overdraw debugging feature to Skia pipelines

Test: Compared to OpenGL pipeline and sanity checked
with understanding of the drawing pipeline.  Also
wrote a unit test.

BUG:32370375

Change-Id: Iab397d21f0def725fa89551d48c764c67fd2bda8
diff --git a/libs/hwui/pipeline/skia/SkiaPipeline.h b/libs/hwui/pipeline/skia/SkiaPipeline.h
index c1c8cbe..c58fedf 100644
--- a/libs/hwui/pipeline/skia/SkiaPipeline.h
+++ b/libs/hwui/pipeline/skia/SkiaPipeline.h
@@ -107,6 +107,18 @@
     renderthread::RenderThread& mRenderThread;
 
 private:
+    void renderFrameImpl(const LayerUpdateQueue& layers, const SkRect& clip,
+            const std::vector< sp<RenderNode> >& nodes, bool opaque, const Rect &contentDrawBounds,
+            SkCanvas* canvas);
+
+    /**
+     *  Debugging feature.  Draws a semi-transparent overlay on each pixel, indicating
+     *  how many times it has been drawn.
+     */
+    void renderOverdraw(const LayerUpdateQueue& layers, const SkRect& clip,
+            const std::vector< sp<RenderNode> >& nodes, const Rect &contentDrawBounds,
+            sk_sp<SkSurface>);
+
     TaskManager mTaskManager;
     std::vector<sk_sp<SkImage>> mPinnedImages;
     static float mLightRadius;