svg external bitmap

This patch enables saving bitmaps in svg as external files, so as to reduce output size and rendering costs, and allow easier post-processing of bitmaps.

Currently only DCT images can be saved as jpg, other images are still embedded in svg.

Its OK to make some local change for me. I note that if `--split-pages` is on, it use decimal page numbers for content, so it is not that consistent? I'd rather have decimal the default, hex as an option.

In SVG an image is defined like this:`<image id="image5" width="4441" height="413" xlink:href="po-16.jpg"/>` in `<defs>`，and is used like this: `<use xlink:href="#image5" transform="matrix(0.48,0,0,0.48,-1048.846,0)"/>`, so the url "po-16.jpg" occurs only once in this page, no matter how many <use> there are.

Thanks for mentioning rvalue reference, I'm not familiar with it yet.