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.

`svg-node-count-limit` is good.

1. -1 as default is ok, but values <= 0 have same effect, since valid xml has at least 1 node.
2. Sure.