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.

Maybe `svg-node-count-limit` or `svg-size-limit`.
Two more things:
- Can you make it -1 by default, and 0 a valid value, because there's another `-limit` option there for temporary files
- Please make sure to write in the man page that this is only an estimate, not the exact number of nodes.