Add new property to debug non-rectangular clip operations

This change adds a new property called "debug.hwui.show_stencil_clip"
that accepts the following values:

- "highlight", colorizes in green any drawing command that's tested
  against a non-rectangular clip region
- "region", shows the non-rectangular clip region in blue every time
  it is used
- "hide", default value, nothing is shown

Change-Id: I83c8602310edc4aaeb8b905371cdd185b17d32b5
diff --git a/libs/hwui/Debug.h b/libs/hwui/Debug.h
index 9a6494f..773fe82 100644
--- a/libs/hwui/Debug.h
+++ b/libs/hwui/Debug.h
@@ -48,6 +48,7 @@
 #define DEBUG_RENDER_BUFFERS 0
 
 // Turn on to make stencil operations easier to debug
+// (writes 255 instead of 1 in the buffer, forces 8 bit stencil)
 #define DEBUG_STENCIL 0
 
 // Turn on to display debug info about 9patch objects