When multiple inputs are rendered through the same mapper, it is possible to
composite overlapping areas correctly. Inputs are connected directly to
the mapper and their parameters (transfer functions, transformations, etc.) are
specified through standard vtkVolume instances. These vtkVolume instances are
to be registered in a special vtkProp3D, vtkMultiVolume.  vtkMultiVolume
represents a 3D space containing a set of vtkVolume instances which could be
but are not necessarily overlapping, this instance computes the bounding-box
containing all of its registered vtkVolumes.

vtkGPUVolumeRayCastMapper prepares and keeps track of the inputs as they are
allowed to be disconnected at any time. vtkOpenGLGPUVRCMapper's internals
particular to an input (vtkVolumeTexture, OpenGL helper instances, etc.) have
been promoted to a separate class vtkVolumeInputHelper. Other internals (such as
ComputeBounds, ComputePointToCellMatrix and related variables) ...