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 mean one call of this function is (0,1), which turned out to be null, the other call with (0,2), with which we will find something. 

Does "this function" refer to `CairoBackgroundRenderer::setMimeData()`? This can't happen, poppler will not do that if (0,1) is deleted, since the call to `XRef::fetch(num=0, gen=1,...)` already returned a null obj.