Fix vertical image bounds when on an half display pixel

The Agg renderer internally has the vertical axis flipped, so the
rounding of the bounds in the vertical direction needs to be round
half down instead of round half up.

Author: ayshih

lib/matplotlib/image.py

@@ -408,7 +408,9 @@ def _make_image(self, A, in_bbox, out_bbox, clip_bbox, magnification=1.0,
         magnified_extents = clipped_bbox.extents * magnification
         if ((not unsampled) and round_to_pixel_border):
             # Round to the nearest output pixel
-            magnified_bbox = Bbox.from_extents((magnified_extents + 0.5).astype(int))
+            x0, x1 = np.floor(magnified_extents[0::2] + 0.5)  # round half up
+            y0, y1 = np.ceil(magnified_extents[1::2] - 0.5)  # round half down
+            magnified_bbox = Bbox.from_extents([x0, y0, x1, y1])
         else:
             magnified_bbox = Bbox.from_extents(magnified_extents)