qt: fix drag and drop in `CompositorPlatform`

When the parent top-level window receive a drag or drop
event, Qt does not propagate the event through the child
windows.

All handling is done in `QQuickWindow`, so if I manually
forward the events to the quick window, drag and drop
works as expected.

modules/gui/qt/maininterface/compositor_platform.cpp

@@ -95,6 +95,8 @@ bool CompositorPlatform::makeMainInterface(MainCtx *mainCtx)
     m_quickWindow->setOpacity(0.0);
     m_quickWindow->setOpacity(1.0);
 
+    m_rootWindow->installEventFilter(this);
+
     m_rootWindow->show();
     m_videoWindow->show();
     m_quickWindow->show();
@@ -153,6 +155,28 @@ QQuickItem *CompositorPlatform::activeFocusItem() const
     return m_quickWindow->activeFocusItem();
 }
 
+bool CompositorPlatform::eventFilter(QObject *watched, QEvent *event)
+{
+    // Forward drag events to the child quick window,
+    // as it is not done automatically by Qt with
+    // nested windows:
+    if (m_quickWindow && watched == m_rootWindow.get())
+    {
+        switch (event->type()) {
+        case QEvent::DragEnter:
+        case QEvent::DragLeave:
+        case QEvent::DragMove:
+        case QEvent::DragResponse:
+        case QEvent::Drop:
+            QApplication::sendEvent(m_quickWindow, event);
+            return true;
+        default:
+            break;
+        };
+    }
+    return false;
+}
+
 int CompositorPlatform::windowEnable(const vlc_window_cfg_t *)
 {
     commonWindowEnable();

modules/gui/qt/maininterface/compositor_platform.hpp

@@ -46,6 +46,8 @@ public:
     Type type() const override;
     QQuickItem * activeFocusItem() const override;
 
+    bool eventFilter(QObject *watched, QEvent *event) override;
+
 private:
     int windowEnable(const vlc_window_cfg_t *) override;
     void windowDisable() override;