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.

I remembered that I used `long` or something to store a whole ref while processing files, and there are also functions converting them, please check `font.cc` and/or `util.cc`