Correctly implement the CLEAR xfermode.

The previous implementation was using glBlendFunc with the parameters
GL_ZERO/GL_ZERO which doesn't work for text, paths and other alpha
sources (anti-aliasing.) The correct implementation is GL_ZERO/
GL_ONE_MINUS_SRC_ALPHA.

Change-Id: I4cca65e57b6a37bbf5a41d382cb0648ee8e11e79
diff --git a/libs/hwui/Debug.h b/libs/hwui/Debug.h
index f74238e..2cdc8c3 100644
--- a/libs/hwui/Debug.h
+++ b/libs/hwui/Debug.h
@@ -37,6 +37,8 @@
 
 // Turn on to display debug info about 9patch objects
 #define DEBUG_PATCHES 0
+// Turn on to "explode" 9patch objects
+#define DEBUG_EXPLODE_PATCHES 0
 // Turn on to display vertex and tex coords data about 9patch objects
 // This flag requires DEBUG_PATCHES to be turned on
 #define DEBUG_PATCHES_VERTICES 0