ENH: Initial take at SmartVolumeMapper support

Copied the vtkSmartVolumeMapper from Rendering/Volume to
Rendering/VolumeOpenGL and Rendering/VolumeOpenGL2 to
provide two different classes for each rendering backend.

Change-Id: Iee1a6d626c7e1f32064d0ee35e34c5ca51ed0bbe
ENH: Removed all vtkVolumeTextureMapper3D refs from OpenGL2 module

The final SmartVolumeMapper just has 2 options : GPURayCast or
FixedPointRayCast rendering.

Change-Id: I902489b53955f6731e7b03ba81b7afc2ce0f95e3
Merge topic 'SmartVolumeMapper_OpenGL2' into master

93af2c18 ENH: Removed all vtkVolumeTextureMapper3D refs from OpenGL2 module
cf497e99 ENH: Initial take at SmartVolumeMapper support