HiDPI-related: Fix center to grab the correct color

Not sure what was wrong with the old formula, but I think it had something to do with using the latest_pb instead of pb, which then broke on HiDPI screens again, making not the center center color the one picked but the top center one.

pick/__main__.py

@@ -1343,7 +1343,7 @@ def magnifier_clicked(self, window, ev):
 
     def get_colour_from_pb(self, pb):
         pixel_data = pb.get_pixels()
-        offset = (pb.get_rowstride() * (self.snapsize[1] / 2)) + ((self.latest_pb.get_rowstride() / self.snapsize[0]) * (self.snapsize[0] / 2))
+        offset = pb.get_rowstride() * pb.get_height() / 2 + (pb.get_rowstride() / 2)
         rgb_vals = tuple([ord(x) for x in pixel_data[offset:offset+3]])
         return rgb_vals