to help debugging [2461567] Home screen redraw messed up

log SF's idea of the front buffer in dumpsys.
diff --git a/libs/surfaceflinger/Layer.h b/libs/surfaceflinger/Layer.h
index 1310ecc..743afb4 100644
--- a/libs/surfaceflinger/Layer.h
+++ b/libs/surfaceflinger/Layer.h
@@ -78,6 +78,8 @@
     inline const sp<FreezeLock>&  getFreezeLock() const { return mFreezeLock; }
     // only for debugging
     inline PixelFormat pixelFormat() const { return mFormat; }
+    // only for debugging
+    inline int getFrontBufferIndex() const { return mFrontBufferIndex; }
 
 private:
     inline sp<GraphicBuffer> getFrontBufferLocked() {