svg2pdf: extract linked images from the SVG for inclusion using pdflatex

This is done because when pdflatex embeds a JPEG or PNG, it reuses the
data from the file, whereas when inkscape generates a PDF from an SVG
that includes a JPEG, inkscape will load the JPEG image data, re-encode it
in some awful uncompressed form and put *that* in the PDF.

TL;DR: pdflatex \includegraphics{*.jpeg} gives a small PDF with a properly
compressed image; inkscape --export-pdf gives a huge PDF with an
uncompressed image.