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.

> If a drawing command tries to fetch (0,1), it will see (0,2) is there, and is aware that (0,1) is removed, and behaves as if (0,1) is null.

I'm pretty sure that this is not correct.