Don't repeat background images when it's not needed

For now, we only avoid the repetition when we have no-repeat for x and y
axes. We could do this when the painting area is smaller than the image and
when the position doesn't require a repetition. But… Who cares?

Fix #238.

There was also a bug in the size of the surface needed to draw images that are
not repeated. Before this commit, the repeat size was the painting size * 2 to
avoid glitches at the boundaries. But we have to be sure to draw at least the
whole image, as the image may be translated before being drawn. The
multiplication is not needed as we don't repeat the image anymore.

Related to #669.

weasyprint/draw.py

@@ -426,7 +426,6 @@ def draw_table_backgrounds(context, page, table, enable_hinting):
 
 
 def draw_background_image(context, layer, image_rendering):
-    # Background image
     if layer.image is None:
         return
 
@@ -439,20 +438,29 @@ def draw_background_image(context, layer, image_rendering):
     image_width, image_height = layer.size
 
     if repeat_x == 'no-repeat':
-        repeat_width = painting_width * 2
+        # We want at least the whole image_width drawn on sub_surface, but we
+        # want to be sure it will not be repeated on the painting_width.
+        repeat_width = max(image_width, painting_width)
     elif repeat_x in ('repeat', 'round'):
+        # We repeat the image each image_width.
         repeat_width = image_width
     else:
         assert repeat_x == 'space'
         n_repeats = math.floor(positioning_width / image_width)
         if n_repeats >= 2:
+            # The repeat width is the whole positioning width with one image
+            # removed, divided by (the number of repeated images - 1). This
+            # way, we get the width of one image + one space. We ignore
+            # background-position for this dimension.
             repeat_width = (positioning_width - image_width) / (n_repeats - 1)
-            position_x = 0  # Ignore background-position for this dimension
+            position_x = 0
         else:
+            # We don't repeat the image.
             repeat_width = image_width
 
+    # Comments above apply here too.
     if repeat_y == 'no-repeat':
-        repeat_height = painting_height * 2
+        repeat_height = max(image_height, painting_height)
     elif repeat_y in ('repeat', 'round'):
         repeat_height = image_height
     else:
@@ -461,7 +469,7 @@ def draw_background_image(context, layer, image_rendering):
         if n_repeats >= 2:
             repeat_height = (
                 positioning_height - image_height) / (n_repeats - 1)
-            position_y = 0  # Ignore background-position for this dimension
+            position_y = 0
         else:
             repeat_height = image_height
 
@@ -471,7 +479,11 @@ def draw_background_image(context, layer, image_rendering):
     sub_context.clip()
     layer.image.draw(sub_context, image_width, image_height, image_rendering)
     pattern = cairo.SurfacePattern(sub_surface)
-    pattern.set_extend(cairo.EXTEND_REPEAT)
+
+    if repeat_x == repeat_y == 'no-repeat':
+        pattern.set_extend(cairo.EXTEND_NONE)
+    else:
+        pattern.set_extend(cairo.EXTEND_REPEAT)
 
     with stacked(context):
         if not layer.unbounded: