Added support for transparent stroke colors (again).

Transparent strokes are rendered by means of transparent vertex colors.
To make this possible, Blender's internal renderer has been slightly
extended to allow transparent vertex colors.  When Material::vcol_alpha
is non-zero, the renderer takes MCol::a into account.

source/blender/freestyle/intern/blender_interface/BlenderStrokeRenderer.h

@@ -37,8 +37,11 @@ protected:
 	Scene* freestyle_scene;
 	Material* material;
 	ListBase objects;
+	float _z, _z_delta;
 
 	void store_object(Object *ob) const;
+
+	float get_stroke_vertex_z(void) const;
 };
 
 #endif // BLENDERSTROKERENDERER_H