DXT3/5 decoders

[flowtsohg]

I am not sure if this repository has any relevance anymore, but it was a great help for me to get me started on DDS DXT images.

I added DXT3/5 decoders, so in case you want them, I am uploading them here.
I also added the color0 <= color1 condition in DXT1 decoding that you didn't put for some reason.

https://gist.github.com/flowtsohg/11306221

Optimizations are welcome :)

[photopea]

Hi, I made my own DXT1 / 3 / 5 / 10 decoders, too :) https://github.com/photopea/UTEX.js

32 MB 8192 x 8192 px texture is parsed in 900 ms! :)

And there is an encoder (compressor), too.