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 it seems that the generation number should be increased if the object with the same object number has been deleted.
To be more precise, in an xref entry marked with 'f', the generation number is "to indicate the generation number to be used the next time an object with that object number is created".