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.

>And about str & ref, I'm not sure if ref is the identifier for str, can we use it to identify each image?

Yes, look at this code path: `Gfx::opXObject -> Gfx::doImage -> OutputDev::drawImage`, ref is pointing to str.