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.

These messages are showing that the required format has not been built into.
So it's the _same_ message, and it'll be easier to maintain that way.
Just a trivial example, if we found a typo, or try to use another message, I hope there's a way to make sure that we won't miss any message - if we have it mentioned multiple times.