I did some more searching myself and found there is now a good solution with Inkscape
I would choose Inkscape because it is available on all 3 requested platforms (Win, Mac and Linux), it is free and fairly easy to use (if you do not do complicated stuff).
Since version 0.91 it is possible to use "symbols" and use symbol libraries of your own.
So, the ...
You don't need an extension because Chrome offers a built-in Full Page Screenshot feature
First, we need to open DevTools inside Chrome. We can open it in different ways:
Main Menu (⋮ three vertical dots) in the top-right corner > More tools > Developer tools.
Keyboard shortcuts: Control+Shift+I in Windows and Linux, or Command+Option+I in Mac.