Linked (non-embedded) pixel graphics in SVG don't work

[mgmax]

Embedded pixel graphics work, but linked pixel graphics don't. See these test files: linked-image.zip

With current master (c695e590b33715fe2287f67836f65510f0552fcb) I get error messages ("Could not parse xlink:href, Could not load image: null"), with newer svgsalamander library (#525) I get no messages, but the image is still missing.

We could work around this in the inkscape extension by embedding all images. There's an existing inkscape extension which is shipped with inkscape and does exactly that.

[t-oster]

That is a good idea imho. If we supported links directly, we should make sure we include the needed files in the PLF format, too, because it would not be portable otherwise.