Fixed: free closure and surface in ~Canvas

src/Canvas.cc

@@ -407,6 +407,8 @@ Canvas::~Canvas() {
   switch (type) {
     case CANVAS_TYPE_PDF:
       closure_destroy((closure_t *) _closure);
+      free(_closure);
+      cairo_surface_destroy(_surface);
       break;
     case CANVAS_TYPE_IMAGE:
       cairo_surface_destroy(_surface);