- Move properties from vol.ren. display node to view node: RaycastTechnique, ExpectedFPS, GPUMemorySize, EstimatedSampleDistance (renamed to VolumeRenderingOversamplingFactor, as the former name was misleading as it does not specify sample distance but an oversampling factor), PerformanceControl (renamed to VolumeRenderingQuality, as it is not directly related to performance, and even the enum was called Quality), SurfaceSmoothing (renamed to VolumeRenderingSurfaceSmoothing).
  Reasons:
  1. Setting these options differently for volumes is not useful, however setting some of them per view could be useful
  2. The new multi volume rendering method has only one common mapper, and some properties cannot be set per volume
  3. It would be nice to expose these properties in Application Settings, to be able to define application defaults. For this, the view node is a better place

Important note: this commit breaks backwards compatibility in terms of loading a scene containing special volume rendering settings, so those settings will not be loaded correctly. However Slicer 4.10 and soon after Slicer 5 is coming up, so this might be a good time to make this step.

- Move GetSampleDistance function from displayable manager to vtkMRMLVolumeRenderingDisplayNode
- Fix bugs related to multi-volume rendering
- Fix vtkSlicerVolumeRenderingLogicTest and fix todo in vtkMRMLVolumeRenderingDisplayable by adding utility functions in the displayable manager

git-svn-id: http://svn.slicer.org/Slicer4/trunk@27240 3bd1e089-480b-0410-8dfb-8563597acbee