Add radial gradient background mode

- Collects different gradient modes under the vtkViewport::GradientModes
enum
- vtkOpenGLRenderer implements textured and gradient backgrounds with
vtkOpenGLQuadHelper instead of 2D actors and mappers. It is simpler and
doesn't involve VTK pipeline updates.
- Updated TestGradientBackground to exercise different gradient modes.
- closes vtk/vtk#19015