Enables volume rendered data in composite ('lut') mode.

This change ensures that we have a decent z raster even if we are volume rendering. With that we can composite the volume's pixels into place and the result will be correct as long as nothing intersects the volume.

Invertible LUT and direct float rendering for blended volume renders won't work because there is no single scalar for the whole path along the ray. mip, avg, and map modes could potentially be made to work.