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.

>But I do see in the spec that 'each object can be identify by the object number and the generation number'

I think here 'object' refers to both deleted and in-use objects.
PDF readers usually don't care about deleted objects; editors might do, for example it may be able to recover deleted objects (I don't known one do this however).