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/Android.mk b/libs/hwui/Android.mk
index 1f37925..962d726 100644
--- a/libs/hwui/Android.mk
+++ b/libs/hwui/Android.mk
@@ -10,6 +10,7 @@
 		thread/TaskManager.cpp \
 		font/CacheTexture.cpp \
 		font/Font.cpp \
+		AmbientShadow.cpp \
 		AssetAtlas.cpp \
 		FontRenderer.cpp \
 		GammaFontRenderer.cpp \
@@ -37,6 +38,7 @@
 		ProgramCache.cpp \
 		RenderBufferCache.cpp \
 		ResourceCache.cpp \
+		ShadowTessellator.cpp \
 		SkiaColorFilter.cpp \
 		SkiaShader.cpp \
 		Snapshot.cpp \