ENH: Use segmentation Opacity attribute only for 2D

Users most often want to tune 2D opacities (fill and outline together), while
keeping 3D opacities at 1.0 (transparency often does not help in 3D and since depth peeling does not work in
the current OpenGL1 backend, semi-transparent results are not even correct).

By using Opacity property for 2D only (fill and outline) we can show/hide segments in slice viewers without impacting 3D visibility.
It's also very convenient that slice view controller adjust this Opacity property.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25466 3bd1e089-480b-0410-8dfb-8563597acbee