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.

What I mean is we could have 3 values for `--svg-embed-bitmap`, 0 for auto, which follows `--embed-image`, and 1 and -1 are to embed images or not.