Override use tag children instead of drawing their references

This change fixes cases when use tags are not supposed to be really drawn, for
example when used to clip paths.

weasyprint/svg/__init__.py

@@ -284,6 +284,13 @@ class Node:
             str(rotate.pop(0) if rotate else original_rotate[-1])
             for i in range(len(self.text)))
 
+    def override_iter(self, iterator):
+        """Override node’s children iterator."""
+        # As special methods are bound to classes and not instances, we have to
+        # create and assign a new type.
+        self.__class__ = type(
+            'Node', (Node,), {'__iter__': lambda _: iterator})
+
 
 class SVG:
     """An SVG document."""
@@ -737,11 +744,7 @@ class SVG:
             if key not in element.attrib:
                 element.attrib[key] = value
         if next(iter(element), None) is None:
-            # Override element’s __iter__ with parent’s __iter__. As special
-            # methods are bound to classes and not instances, we have to create
-            # and assign a new type.
-            element.__class__ = type(
-                'Node', (Node,), {'__iter__': lambda self: parent.__iter__()})
+            element.override_iter(parent.__iter__())
 
     def calculate_bounding_box(self, node, font_size, stroke=True):
         """Calculate the bounding box of a node."""

weasyprint/svg/defs.py

@@ -60,10 +60,8 @@ def use(svg, node, font_size):
             if key not in tree.attrib:
                 tree.attrib[key] = value
 
-    svg.stream.push_state()
+    node.override_iter(iter((tree,)))
     svg.stream.transform(e=x, f=y)
-    svg.draw_node(tree, font_size)
-    svg.stream.pop_state()
 
 
 def draw_gradient_or_pattern(svg, node, name, font_size, opacity, stroke):