vtkLightBoxRendererManager - Fix warning related to missing active camera

This issue was occurring after (1) adding a markup and (2) registering
a custom Slicer layout having at least on Slice view.

It addresses the following warning:

  WorldToView: no active camera, cannot compute world to view, returning 0,0,0

The "no active camera" warning/issue is also discussed in Slicer issue
#1690 but note that this commit does NOT address that problem.
Merge branch 'associate-camera-with-slice-view'

* associate-camera-with-slice-view:
  vtkLightBoxRendererManager - Fix warning related to missing active camera