Fix, work-around, and debug printf()'s.  Got DXT5 working.

I now have the correct block size for DXT3/5 (16 bytes instead of 8).  My
understanding of the "srcRowStride" parameter of the external library was
wrong; so I worked-around it (off by a factor of 4).  I added temporary
debug/printf code to make debugging faster.
Put #ifdef's around the debug printf() statements.
Better algorithm for stride.