Split Mesh:attachAttribute(name, nil) out into Mesh:detachAttribute. Fix attachAttribute.

Don’t let me push commits on a friday night…

--HG--
branch : minor

src/modules/graphics/opengl/Mesh.h

@@ -139,6 +139,7 @@ public:
 	 * will be used when drawing this Mesh.
 	 **/
 	void attachAttribute(const std::string &name, Mesh *mesh, const std::string &attachname);
+	bool detachAttribute(const std::string &name);
 
 	void *mapVertexData();
 	void unmapVertexData(size_t modifiedoffset = 0, size_t modifiedsize = -1);