the change is mostly a slight improvement to image quality over the prior encoders:
incorporates slight dithering, more like the DXT5 encoder (and is based on the code from the DXT5 encoder);
uses 4 colors when possible for a block, only falling back to 3-colors when transparent pixels are present;
included are a few test images (originals, and as-encoded with this encoder).
(ignore duplicate image, site apparently wont allow removing it...).