Failure to perform a vertical flip on S3TC-DXTC now simply leaves the original image instead of corrupting it.
Image.cpp was sometimes performing a "normal" (= for uncompressed images) vertical flip on S3TC-DXTC images, producing weird results.
Actually, code was trying a "DXTC vertical flip" and relied on the result to call a "normal vertical flip". But when the "DXTC v-flip" encounters an error, this is is not necessarily because the image is not S3TC (ex: unhandled image dimensions)!
So now the code simply does "if dxtc, then flip_dxtc; else flip_normal;".
Note from Robert Osfield, moved the isDXT function into the dxt_tool file and namespace.
