lol
My friend Sam shared with me
this link (via email) which I only did some rudimentary vetting, so user beware. They have a lot of other cool online tools as well. They do tag the result (lower left corner) so, if you don't like this, you could buffer your target with some blank info. They offer SVG export to, but you have to contact the author and I'm sure you'll have to pay for this privilege. G'MIC does have a preset called packed sprites as you guys know, but this online tool is color aware and appears to offer more precise output.
![Smile :)](./images/smilies/icon_smile.gif)