Don’t limit the opacity groups to the original box size

It doesn’t work when elements are displayed outside or when the box is
transformed.

Author: liZe

tests/draw/test_transform.py

@@ -285,3 +285,25 @@ def test_2d_transform_13(assert_pixels):
               font-size: 0 }
       </style>
       <div><img src="pattern.png"></div>''')
+
+
+@assert_no_logs
+def test_2d_transform_opacity(assert_pixels):
+    assert_pixels('''
+        __________
+        __________
+        __________
+        _ss_______
+        _ss_______
+        __________
+        __________
+        __________
+        __________
+        __________
+    ''', '''
+      <style>
+        @page { size: 10px; margin: 2px; }
+        div { transform: translate(-1px, 1px); background: red;
+              height: 2px; width: 2px; opacity: 0.5 }
+      </style>
+      <div></div>''')

weasyprint/draw.py

@@ -108,10 +108,7 @@ def draw_stacking_context(stream, stacking_context):
 
         if box.style['opacity'] < 1:
             original_stream = stream
-            stream = stream.add_group([
-                box.border_box_x(), box.border_box_y(),
-                box.border_box_x() + box.border_width(),
-                box.border_box_y() + box.border_height()])
+            stream = stream.add_group(stream.page_rectangle)
 
         if box.transformation_matrix:
             if box.transformation_matrix.determinant: