Tessellate on worker threads

Tessellate and cache (where possible) shadow and round rect
tessellation tasks.

Change-Id: I2cfda8e11d83d51ea74af871235cf26e8f831d40
diff --git a/libs/hwui/AmbientShadow.h b/libs/hwui/AmbientShadow.h
index 451bfbe..68df246 100644
--- a/libs/hwui/AmbientShadow.h
+++ b/libs/hwui/AmbientShadow.h
@@ -35,7 +35,7 @@
  */
 class AmbientShadow {
 public:
-    static VertexBufferMode createAmbientShadow(bool isCasterOpaque, const Vector3* poly,
+    static void createAmbientShadow(bool isCasterOpaque, const Vector3* poly,
             int polyLength, const Vector3& centroid3d, float heightFactor,
             float geomFactor, VertexBuffer& shadowVertexBuffer);