x11: Always synthesize size events for external windows

External windows may call this to update the renderer size, but not pump SDL events, either by syncing or running an event loop, so the size event always needs to be synthesized for external windows. If it is wrong, the true size will be sent if/when events are processed.

src/video/x11/SDL_x11window.c

@@ -1324,6 +1324,14 @@ void X11_SetWindowSize(SDL_VideoDevice *_this, SDL_Window *window)
         data->pending_operation |= X11_PENDING_OP_RESIZE;
         X11_XResizeWindow(display, data->xwindow, data->expected.w, data->expected.h);
     }
+
+    /* External windows may call this to update the renderer size, but not pump SDL events,
+     * so the size event needs to be synthesized for external windows. If it is wrong, the
+     * true size will be sent if/when events are processed.
+     */
+    if (window->flags & SDL_WINDOW_EXTERNAL) {
+        SDL_SendWindowEvent(window, SDL_EVENT_WINDOW_RESIZED, window->pending.w, window->pending.h);
+    }
 }
 
 bool X11_GetWindowBordersSize(SDL_VideoDevice *_this, SDL_Window *window, int *top, int *left, int *bottom, int *right)