Guard QGL2PaintEngineEx/QGLPaintDevice against using unset context

QGL2PaintEngineEx::end() calls endPaint() on the device, which may end up
calling QGLContext::swapBuffers() on the context, a call which we have
documented to require makeCurrent() for any further GL commands:

  Call this to finish a frame of OpenGL rendering, and make sure to
  call makeCurrent() again before issuing any further OpenGL commands,
  for example as part of a new frame.

QOpenGLContext also documents that:

  To be able to support certain platforms, QOpenGLContext requires that
  you call makeCurrent() again before starting rendering a new frame,
  after calling swapBuffers().

As a consequence, we need to guard the GL usage in QGLPaintDevice::endPaint(),
as well as the GL reset in QGL2PaintEngineEx::end().

Change-Id: Ib66e91cce12fbaea32ed3c30141150fa15907187
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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