vtkViewLayout: Fix stereo in cave/tile displays

When running pvserver in a CAVE environment with --stereo --stereo-type="Crystal Eyes", stereo was not working and the display on the server flashed as if one of the eyes was the correct image and the other was just white. Just using a simple vtk python script, crystal eyes stereo worked fine, so seemingly the fbos managed by vtkOpenGLRenderWindow contained the correct images. This change blits to hardware from the display fbo rather than the render fbo, which is where the color attachments for the two eyes actually seem to live.