Now that I'm playing with svg I've found that exported path from The Gimp have duplicated points…
And that modified svg from Inkscape have relatives coordinates…
I didn't find an SVG parser library yet. One which seems to extract the path nicely… There's some chunk of code on github, though:https://github.com/ippa/svg_parser
a bit old, very limited parser can't read path from Gimp because of XPATH expression limited to Inkscape, use hpricot (deprecated) https://github.com/rfrankel/Parse-SVG-Paths
not tested yet, more complete, more recent libhttp://www.inkscapeforum.com/viewtopic.php?f=5&t=4601
many be some hint or lib here SVG Path specification and lib 2Geom.
Of course my click and pointing shape editor in my own ruby code, would have been probably working already, if I've not explored the SVG way…
But application like Inkscape and Gimp, are of course much more powerful.
I'm still looking for game editor's code if someone have seen a base with load/save/export features.