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 mean when the first `draw` instruction appears the first img is already rendered.
I just read a few sections in the PDF spec, and did not see what you mentioned. But I do see that both the id and gen number can uniquely identify an object.
Can you show me the relevant sections?