Calculate and show Ambient shadow.

Basically we compute the shadow as a strip of triangles, whose alpha value
is the strength of the shadow.
We use the normal to extend the geometry.
And we use static function and try to avoid new/malloc in the computation.

Change-Id: I382286f1cad351bd5ff983f76f446c075819dcaf
diff --git a/libs/hwui/Debug.h b/libs/hwui/Debug.h
index 79afad1..f619205 100644
--- a/libs/hwui/Debug.h
+++ b/libs/hwui/Debug.h
@@ -88,6 +88,9 @@
 // Turn on to enable 3D support in the renderer (off by default until API for control exists)
 #define DEBUG_ENABLE_3D 0
 
+// Turn on to enable debugging shadow
+#define DEBUG_SHADOW 0
+
 #if DEBUG_INIT
     #define INIT_LOGD(...) ALOGD(__VA_ARGS__)
 #else