How do get DXT1 alpha dither?

[tomspilman]

Hey @castano .

I am trying to get the same alpha dither behavior as i've seen in other DXT1 compressors:

(progressive blocks of alpha at 90%,80%,70%,60%,etc...)

I tried using the SetQuantization API with alphaDither, but it doesn't seem to be supported for DXT:

    if (compressionOptions.enableAlphaDithering) {
        if (compressionOptions.format == Format_RGB) {
            img.quantize(3, compressionOptions.asize, true, true);
        }
    }

https://github.com/castano/nvidia-texture-tools/blob/master/src/nvtt/Context.cpp#L531

Is this something that was overlooked and needs a fix?