Adds an ActiveState ivar to vtkOpenGLGL2PSHelper to replace
Capturing. There are three possible states that vectorized
mappers/actors need to handle:
- Inactive: Render as normal. This really shouldn't happen.
- Background: Render pass to build the rasterized background image
              containing props that cannot be vectorized. Any
              vectorizable geometry should not be drawn.
- Capture: Render vectorizable props using the gl2ps APIs.