Inkscape is again used for .svg->.png conversion

The crashes in Inkscape only happen when converting very large
.svg files. So the calling function (e.g. the snapshot plotter
for physiosets) should take care of never using Inkscape functions
to convert such large files. The solution that was previously
adopted prevented the generation of thumbnails under Windows
which has a major effect on the usability of the generated HTML
reports.