The dashboards mark a test as failing when an override is not found,
and the message is not relevant when using the OpenGL2 backend that
does not implement the painter API. Moved the initialization to the
OpenGL 1 variant of vtkOpenGLRenderWindow, and set NULL in the
parent class.

Change-Id: Ibc5bad78c9d9d286b93437ff1400dc163d1940e4