add ability to export prebaked LUTs instead of value rasters

With LUT, export is faster and file size is smaller, and the result is even easier to interpret than value renders. It is a potential path toward bringing back volume rendered exports.

The choice controllable, and you pick either all value or all prebaked lut. We make a not of the choice in the value_type entry. With lut we don't export luminance to cut down on time and space.

The viewer works as is, but we could update it to not show the colormap choice when showing LUTs.