Replaced broken decompression code with two calls to DXT5 decompression. This should fix some issues with normal maps as they usually are stored as RG Textures and were decompressed incorrectly.
This Commit fixes the corrupted file preview described in #12220. Added DecompressColourBc5 function to squish.
Sources are untouched, tarball from https://sourceforge.net/projects/libsquish