X11: Improve window handle race condition

The non-root parent window owned by the WM could be destroyed before we
process the ConfigureNotify event using the cached parent handle.

Bug was found by unmapping a decorated window.

This like all uses of the Xlib error handler is not thread safe and
there is nothing we can do about that short of moving to XCB.

Fixes #1633.

src/x11_window.c

@@ -1547,6 +1547,8 @@ static void processEvent(XEvent *event)
             //       the position into root (screen) coordinates
             if (!event->xany.send_event && window->x11.parent != _glfw.x11.root)
             {
+                _glfwGrabErrorHandlerX11();
+
                 Window dummy;
                 XTranslateCoordinates(_glfw.x11.display,
                                       window->x11.parent,
@@ -1554,6 +1556,10 @@ static void processEvent(XEvent *event)
                                       xpos, ypos,
                                       &xpos, &ypos,
                                       &dummy);
+
+                _glfwReleaseErrorHandlerX11();
+                if (_glfw.x11.errorCode == BadWindow)
+                    return;
             }
 
             if (xpos != window->x11.xpos || ypos != window->x11.ypos)