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.

I see.. Let me see if there are better ways to do that. And there are actually other versions of CairoOutputDev in the repository, would be a mess if we want to maintain all of them.