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 can only find this paragraph

> Except for object number 0, all objects in the cross-reference table shall initially have generation numbers of 0.
When an indirect object is deleted, its cross-reference entry shall be marked free and it shall be added to the
linked list of free entries. The entry’s generation number shall be incremented by 1 to indicate the generation
number to be used the next time an object with that object number is created. Thus, each time the entry is
reused, it is given a new generation number. The maximum generation number is 65,535; when a cross-
reference entry reaches this value, it shall never be reused. 

So I interpret it as reusing entries instead of reusing object. 

I do see in the spec that you can directly update a (id,gen) by simply define it again, in that case it won't make sense to use a large generation number for updating.

Let me see if I can find other references.