Verify ASTC compressed textures in .ktx containers.

Is there a tool that generates astc in ktx containers? Perhaps we could add support for .astc files (but this is bad in the sense that we do not need yet another file format, and that each mipmap level would be a different file which we cannot handle - so loading pre-generated mipmaps would not be supported here)

-> pvrtextoolcli can apparently do this. (astc in ktx, including mipmaps, for instance: pvrtextoolcli -i d3.png -o d3_astc_4x4.ktx -f ASTC_4x4,UB,lRGB -m 10)