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.

Like in SVG, images can also be embedded or not in HTML, and that's what `--embed-image` controls.
So imagine a user specify a value to `--embed-image`, probably we should also do the same thing for images in SVG, unless a value is explicitly specified for `--svg-embed-bitmap`. 
Does it make sense?