Patch to fix vorbis docbook compilation on trunk

Currently, building the docs on trunk is broken. The attached patch corrects this. First off, I replace some PNGs with recompressed versions (I used advpng -z4; if you want, you can just run that on all PNG files in doc/ instead of copying the ones I attached).

Then I changed Makefile.am to use fop instead of pdfxmltex to build the PDF file. This works pretty well (with the exception of the PNG weirdness I mentioned).

As far as I can tell, this is one of the major blockers for new libvorbis 1.2.x releases.