svcdsub: fix the palette reading order

The palette is written as YUVA octets. And the vcdimager documentation [1] says the is stored as YUVT:

These are formated as Y, U, V, and T (transparancy) each a single byte. T=0 is fully transparent, and 255 is fully opaque.

It's just more logical to write the index in order and read it as YCbCrT.

[1] https://github.com/rocky/vcdimager/blob/master/docs/svcd-ogt-subtitles.pod