Avoid duplicate StillRender() calls after interactive render.

When LOD was enabled and user interacted with the scene, we ended up with 2
calls to StillRender(). That was because the logic in
vtkPVGenericRenderWindowInteractor was redundantly calling StillRender(). Fixed
that.

Change-Id: Ib0ebafcb17d59e50bc5335de8a6b9c3cbed5563f
Merge topic 'fix_double_still_renders' into next

c36f973 Avoid duplicate StillRender() calls after interactive render.