For OpenGL2, the vtkOpenGLImageSliceMapper:RenderPolygon was being
called even if the slice specified by vtkImageResliceMapper was
beyond the volume bounds.  This caused a regression: specifically,
when the slice moved out-of-bounds the old polygon would be reused
even though no polygon should be displayed at all.  The fix is for
RenderPolygon to return early if no polygon should be displayed.

Closes #17265