Add inline comment about lines that need to match shift calculation

(#1322)

galsim/gsobject.py

@@ -1225,6 +1225,8 @@ def _get_new_bounds(self, image, nx, ny, bounds, center):
         elif nx is not None and ny is not None:
             b = BoundsI(1,nx,1,ny)
             if center is not None:
+                # Note: this needs to match the corresponding calculation in _setup_image
+                # where we shift the image center after making a new image with nx,ny.
                 b = b.shift(_PositionI(np.floor(center.x+0.5-b.true_center.x),
                                        np.floor(center.y+0.5-b.true_center.y)))
             return b