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.

Oh, I mean `Always embed bitmap files in svg`. You don't need to write full description here.
And I forgot to mention that please add new items in the manpage, with full description.

Possibly like this:
`if set to 0, export bitmap files in svg when possible; otherwise always embed them`.

I think it may make more sense if it follows the `embed-image` option.