window: app frame split from anbox window.

Press app close button and don't release, if click point is on the other app's titlebar,
drag will split app frame from anbox window on the second window.

src/anbox/platform/sdl/platform.cpp

@@ -313,17 +313,24 @@ void Platform::process_input_event(const SDL_Event &event) {
   std::int32_t x = 0;
   std::int32_t y = 0;
 
+  bool bFind = false;
   switch (event.type) {
     // Mouse
     case SDL_MOUSEBUTTONDOWN:
       for (auto &iter : windows_) {
         if (auto w = iter.second.lock()) {
-          if (w->window_id() == event.window.windowID &&
-                  w->title_event_filter(event.button.x, event.button.y)) {
-            return;
+          if (w->window_id() == event.window.windowID) {
+            if (w->title_event_filter(event.button.x, event.button.y)) {
+              return;
+            }
+            bFind = true;
+            break;
           }
         }
       }
+      if (!bFind) {
+        return;
+      }
       if (config_.no_touch_emulation) {
         mouse_events.push_back({EV_KEY, BTN_LEFT, 1});
       } else {