Add another missing window-border recalculation.

This one is triggered by the following sequence:
 - fill up the terminal window with text ('ls -l /dev' or similar)
 - Win+Right then Win+Up to snap to the top right quadrant
 - interactively drag away from the top right quadrant with the title
   bar, which returns the window to its pre-snap size.

After the snap, the window border will have been recomputed to take
account of the window size not being an integer number of character
cells. So it needs recomputing back again the next time the window
size changes to something that _is_ an integer number - which happens
(or rather, we process it in a deferred manner) at the EXITSIZEMOVE.
So that's where we need to recompute the border (again).

windows/window.c

@@ -2815,6 +2815,7 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
                       conf_get_int(conf, CONF_savelines));
             InvalidateRect(hwnd, NULL, true);
         }
+        recompute_window_offset();
         break;
       case WM_SIZING:
         /*