Simplify/fix save_diff_image.

- The previous `*255*10` factor was necessary when using the old _png
  module to read images, which read them as 0-1 floats, the 255 factor
  was needed prior to conversion to uint8.  Now we use pillow everywhere
  in matplotlib.testing.compare and work with uint8 throughout, so the
  extra 255 is incorrect (it basically caused any slight difference to
  be fully saturated).
- We don't use _png to write the diffs anymore, and pillow handles RGB
  just fine, so we don't need to add an alpha channel if it's not there.

Author: anntzer

lib/matplotlib/testing/compare.py

@@ -506,21 +506,13 @@ def save_diff_image(expected, actual, output):
         raise ImageComparisonFailure(
             "Image sizes do not match expected size: {} "
             "actual size {}".format(expected_image.shape, actual_image.shape))
-    abs_diff_image = np.abs(expected_image - actual_image)
+    abs_diff = np.abs(expected_image - actual_image)
 
     # expand differences in luminance domain
-    abs_diff_image *= 255 * 10
-    save_image_np = np.clip(abs_diff_image, 0, 255).astype(np.uint8)
-    height, width, depth = save_image_np.shape
+    abs_diff *= 10
+    abs_diff = np.clip(abs_diff, 0, 255).astype(np.uint8)
 
-    # The PDF renderer doesn't produce an alpha channel, but the
-    # matplotlib PNG writer requires one, so expand the array
-    if depth == 3:
-        with_alpha = np.empty((height, width, 4), dtype=np.uint8)
-        with_alpha[:, :, 0:3] = save_image_np
-        save_image_np = with_alpha
+    if abs_diff.shape[2] == 4:  # Hard-code the alpha channel to fully solid
+        abs_diff[:, :, 3] = 255
 
-    # Hard-code the alpha channel to fully solid
-    save_image_np[:, :, 3] = 255
-
-    Image.fromarray(save_image_np).save(output, format="png")
+    Image.fromarray(abs_diff).save(output, format="png")