vo_opengl_cb: add a function to report vsync time

And also let vo.c know of it.

Currently, this does not help much, but will facilitate future
improvements.

DOCS/client_api_examples/qml_direct/main.cpp

@@ -96,6 +96,8 @@ void MpvObject::handleWindowChanged(QQuickWindow *win)
             this, &MpvObject::sync, Qt::DirectConnection);
     connect(win, &QQuickWindow::sceneGraphInvalidated,
             this, &MpvObject::cleanup, Qt::DirectConnection);
+    connect(win, &QQuickWindow::frameSwapped,
+            this, &MpvObject::swapped, Qt::DirectConnection);
     win->setClearBeforeRendering(false);
 }
 
@@ -110,6 +112,11 @@ void MpvObject::sync()
     renderer->size = window()->size() * window()->devicePixelRatio();
 }
 
+void MpvObject::swapped()
+{
+    mpv_opengl_cb_report_flip(mpv_gl, 0);
+}
+
 void MpvObject::cleanup()
 {
     if (renderer) {

DOCS/client_api_examples/qml_direct/main.h

@@ -37,6 +37,7 @@ public:
 public slots:
     void command(const QVariant& params);
     void sync();
+    void swapped();
     void cleanup();
 signals:
     void onUpdate();