video: Explicitly disallow setting the parent of a window to itself

Doing so causes a cycle in the window hierarchy tree graph, which leads to infinite recursion when destroying the windows.

Author: Kontrabant

src/video/SDL_video.c

@@ -3672,6 +3672,10 @@ bool SDL_SetWindowParent(SDL_Window *window, SDL_Window *parent)
         CHECK_WINDOW_NOT_POPUP(parent, false);
     }
 
+    if (window == parent) {
+        return SDL_SetError("Cannot set the parent of a window to itself.");
+    }
+
     if (!_this->SetWindowParent) {
         return SDL_Unsupported();
     }