Expose a getter for the QSGEnginePrivate

This allows getting its underlying QSGContext and QSGRenderContext.

Change-Id: If0773d09b6faa360e706949757db1a6ddbe244e1
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/quick/scenegraph/util/qsgengine_p.h

@@ -45,8 +45,8 @@ class QSGRenderContext;
 class QSGEnginePrivate : public QObjectPrivate
 {
     Q_DECLARE_PUBLIC(QSGEngine)
-
 public:
+    static QSGEnginePrivate *get(QSGEngine *engine) { return engine->d_func(); }
     QSGEnginePrivate();
 
     QScopedPointer<QSGContext> sgContext;