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.

If I understand the PDF spec right, only one object with a certain obj number can be active in a PDF file, that is the one with maximum generation number; other objects with the same obj number and smaller generation numbers must have been marked as removed in xref table, and don't contribute to the rendering result.

So, obj number is enough to identify any active object in PDF.