Fix vertex_surface conversion in graph_draw()

src/graph_tool/draw/cairo_draw.py

@@ -156,7 +156,6 @@ def surface_from_prop(surface):
                 if surface[v] not in surface_map:
                     sfc = gen_surface(surface[v])
                     surface_map[surface[v]] = sfc
-                else:
                 prop[v] = surface_map[surface[v]]
             elif surface.value_type() == "python::object":
                 if isinstance(surface[v], cairo.Surface):