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.

Not sure if we can rely on it, the API is for (their) internal use, and is likely to change in the future. (although maybe not for `gfree` here)