Fix behavior of stencil clipping within clearLayerRegions

bug:16376960

The draw within clearLayerRegions should never be affected by the
current stencil clip, since it's just ensuring that the content of the
layer is cleared, and not doing real content drawing.

Also, add optional verbose GL event logging

Change-Id: I538b1bc631fc091340b76e12db6af0c219851b57
diff --git a/libs/hwui/StatefulBaseRenderer.cpp b/libs/hwui/StatefulBaseRenderer.cpp
index 140c6e8..473005c 100644
--- a/libs/hwui/StatefulBaseRenderer.cpp
+++ b/libs/hwui/StatefulBaseRenderer.cpp
@@ -178,6 +178,7 @@
     SkRegion region;
     region.setPath(transformed, clip);
 
+    // region is the transformed input path, masked by the previous clip
     mDirtyClip |= mSnapshot->clipRegionTransformed(region, op);
     return !mSnapshot->clipRect->isEmpty();
 }