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.

`--embed-image` and `--svg-embed-bitmap` now are orthogonal, both 4 combinations are valid (`--embed-image 1 --svg-embed-bitmap 0` seems making some browsers unhappy, need more study).

I prefer keep them orthogonal, 'auto' looks tricky for me.