Summary of bug description
   * legacy mitkVolumeDataVtkMapper3D had 3 LOD levels, but used the same rendering mode for 2 levels
   * modify RenderingManager to set maximum LOD to 1 ( ^= 2 LOD levels)
 
Cause of the bug
When integrating MIP-Projection Rendering Mode in mitkVolumeDataVtkMapper3D,
it was unsupported in the vtkVolumeTextureMapper2D mapper, and he was disabled,
but still his LOD level was present.

Proposed solution
   * modify RenderingManager to set maximum LOD to 1 ( ^= 2 LOD levels)

Affected classes
   * mitk::RenderingManager
   * mitk::VolumeDataVtkMapper3D

How will the bugfix get tested?
   manual test