Reparent windows to their current position on unmanage

When unmanaging a window we need to reparent it back to the root
window. With this patch we do so at the current position of the
container such that we don't throw away the geometry position.

This fixes cases where clients withdraw a window and reparent it
later on, expecting to keep the same geometry (in particular dock
clients) but then end up on a wrong output.

fixes #3819

src/tree.c

@@ -229,7 +229,7 @@ bool tree_close_internal(Con *con, kill_window_t kill_window, bool dont_kill_par
             xcb_change_window_attributes(conn, con->window->id,
                                          XCB_CW_EVENT_MASK, (uint32_t[]){XCB_NONE});
             xcb_unmap_window(conn, con->window->id);
-            cookie = xcb_reparent_window(conn, con->window->id, root, 0, 0);
+            cookie = xcb_reparent_window(conn, con->window->id, root, con->rect.x, con->rect.y);
 
             /* Ignore X11 errors for the ReparentWindow request.
              * X11 Errors are returned when the window was already destroyed */