OpenGLPaintRedirector updates textures directly

Ownership of decoration textures is moved from SceneOpenGL::Window to
OpenGLPaintRedirector. The PaintRedirector is responsible for updating
the textures whenever they change. For this GLTexture is extended by an
update(QImage, QPoint) method which uses glTexSubImage2D to update only
the changed parts.

The big advantage compared to before is that if e.g. only a button is
animated only the button part is updated instead of the complete deco
part.