Added MultiBlockVolumeMapper demo

An example illustrating the use of the vtkMultiBlockVolumeMapper to render a vtkMultiBlockDataSet.

Added it to the wasm-exclude list as well - since all the other volume rendering examples are on there I suppose that volume rendering isn't supported (yet?) in wasm.

I haven't tried adding images in commits as that failed with my other merge request, and instead provide them here:

An image of how the demo looks right after starting up:

An image of a little bit rotated volume (looks better to me but I don't know whether its suitable?):