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.

"7.5.4 Cross-Reference Table" and "H.7 Updating Example" in "Document management — Portable document format — Part 1: PDF 1.7"