Copied QOpenGLFunctions Core objects crash in destructor

When a QOpenGLFunctions Core object is copied, we encountered a crash during the destruction of the object.

Christian Ehrlicher suggested me to report this issue to you:

https://forum.qt.io/topic/95927/crash-when-copying-qopenglfunctions-core-objects

Example (from the forum):

  #include <QOpenGLFunctions_4_5_Core>

  void CopyCoreObjects()
  {
    {
      QOpenGLFunctions_4_5_Core coreObject1;
      {
        QOpenGLFunctions_4_5_Core coreObject2 = coreObject1;
      }  // OK, calls ~QOpenGLFunctions_4_5_Core().

    } // Crash!
  }

This issue appears to exist for all QOpenGLFunctions Core classes (from QOpenGLFunctions_3_2_Core to QOpenGLFunctions_4_5_Core).

The easiest way to solve this issue seems to be just to prevent accidental copying of QOpenGLFunctions Core objects, by marking the copy-constructor and the assignment-operator of the Core classes as deleted member functions (using C++11 "= delete" syntax).