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 agree, and it does not conflict with my example. So (0,1) and (0,2) may not exist at the same time, but (0,1) can be used before (0,2) has been created.