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 hope I can find an example for this.

I just send 2 pdf to your gmail. In `small-image-del.pdf` I mark the image (7, 0) 'f', so it is not rendered.
```
xref
0 1
0000000007 65535 f 
7 1
0000000000 00001 f 
```