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.

Just to make sure (again) that DCT means jpg ?
Do you think it's a good idea to change `build_bitmap_path` to `build_jpg_path` or something? 
Because at one moment I was worrying about if this needs linking with libjpeg (which actually does not, right?)