RGBA Component Mapping not working with Multi Volume

[psavery]

Currently, if you have one volume module that is using RGBA component mapping, and you create another volume module, you'll get error messages like this:

(  29.443s) [paraview        ]vtkGPUVolumeRayCastMapp:443    ERR| vtkOpenGLGPUVolumeRayCastMapper (0x56129d88b560): If IndependentComponents is Off in the volume property, then the data must have either 2 or 4 component scalars. The input data has 3 component(s).

This happens because if you create more than one volume module, multi volume becomes automatically enabled, and when you are using RGBA component mapping, the mapper uses a different input (an internal data object in the module) than the default input for volume modules.

Perhaps we should either:

1. Not allow multi volumes and RGBA component mapping to be done simultaneously
2. Get the multi volumes to use the module's internal data object when the module is using RGBA component mapping