Do not store the mouse position we calculate as the start of a drag back

into the mouse event that later code uses, it has been adjusted and they
should use the original position. GitHub issue 1710.

window-copy.c

@@ -3505,6 +3505,8 @@ window_copy_start_drag(struct client *c, struct mouse_event *m)
 	window_copy_update_cursor(wme, x, y);
 	window_copy_start_selection(wme);
 	window_copy_redraw_screen(wme);
+
+	window_copy_drag_update(c, m);
 }
 
 static void