x11: check if the X server honored our XMoveWindow() call (thanks, R.E. Rust!).

This can happen if a window is still grabbed when we try to move it, or if
the X11 ecosystem is just in a bad mood, I guess.

This makes sure that SDL will report the correct position for a window;
otherwise, SDL_GetWindowPosition will just report whatever the last
SDL_SetWindowPosition call requested, even if the window didn't actually move.

Fixes Bugzilla #4646.

src/video/x11/SDL_x11window.c

@@ -805,9 +805,24 @@ X11_SetWindowPosition(_THIS, SDL_Window * window)
 {
     SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
     Display *display = data->videodata->display;
+    unsigned int childCount;
+    Window childReturn, root, parent;
+    Window* children;
+    XWindowAttributes attrs;
 
+    /*Attempt to move the window*/
     X11_XMoveWindow(display, data->xwindow, window->x - data->border_left, window->y - data->border_top);
     X11_XFlush(display);
+
+    /*If the window is not moved, then the coordinates on the window structure are out of sync, so we
+      update them here. */
+    X11_XQueryTree(display, data->xwindow, &root, &parent, &children, &childCount);
+    X11_XGetWindowAttributes(display, data->xwindow, &attrs);
+    X11_XTranslateCoordinates(display,
+                              parent, DefaultRootWindow(display),
+                              attrs.x, attrs.y,
+                              &window->x, &window->y,
+                              &childReturn);
 }
 
 void