Combine transferFunction1D color + opacity arrays

[jeffamstutz]

After discussing how the colormap sampler idea could entirely be implemented by image1D samplers (where color + opacity are in a single image array) and due to the fact that there are not control points in the transferFunction1D volume, I think it makes sense to combine color + opacity arrays into a single FLOAT32_VEC4 array. This simplifies applications by allowing them to have a single color map representation that is used to color map both surfaces and volumes.

[johguenther]

Even without control points the color and opacity arrays can be of different size.

For materials we have a similar situation: having color and opacity separate, but if color if it is a SAMPLER additionally use the 4th channel as opacity as well.

So, have here both as well (opacity and color as VEC3 and VEC4)?