Improve direct rendering API in QWaylandCompositor

Result of brain storming:

class QWaylandCompositor {
bool pageFlipSurface(QWaylandSurface *surface, QOpenGLContext *context);

/*
// example usage from qwindow-compositor
if (!QWaylandCompositor::pageFlipSurface(myFullscreenSurface, context))

{ // fall back to compositing context->makeCurrent(window); composite(); context->swapBuffers(window); }

*/
};

class WaylandSurfaceItem : PageFlippableQuickItem
{
public:
bool post(QOpenGLContext *context)

{ // is context really necessary? return m_compositor->pageFlipSurface(m_waylandSurface, context); }

/*
// scene graph code
if (m_fullscreenPageFlippableItem && m_fullscreenPageFlippableItem->post(m_context))
return;
m_context->makeCurrent(m_window);
renderScreneGraph();
m_context->swapBuffers(m_window);
*/
};

The problem with setDirectRenderSurface() is that the compositor doesn't know whether the direct render surface was successfully set and whether it should go on compositing or not.