Cycles: Calculate face normal on the fly.

Instead of pre-calculation and storage, we now calculate the face normal during render.
This gives a small slowdown (~1%) but decreases memory usage, which is especially important for GPUs,
where you have limited VRAM.

Part of my GSoC 2014.

intern/cycles/render/mesh.h

@@ -120,7 +120,7 @@ public:
 	void add_face_normals();
 	void add_vertex_normals();
 
-	void pack_normals(Scene *scene, float4 *normal, float4 *vnormal);
+	void pack_normals(Scene *scene, float *shader, float4 *vnormal);
 	void pack_verts(float4 *tri_verts, float4 *tri_vindex, size_t vert_offset);
 	void pack_curves(Scene *scene, float4 *curve_key_co, float4 *curve_data, size_t curvekey_offset);
 	void compute_bvh(SceneParams *params, Progress *progress, int n, int total);