Bill Kendrick (the guy that made defendguin, among other things) was working on something called libcrtxy for awhile. Not sure what the status is these days and I never tried to use it, but it looks like it might be kind of what you're looking for.
http://libcrtxy.sourceforge.net/Edit: If you can draw lines, and you're talking strictly 2D, it's trivial to zoom in and out (simple scaling and translation in x and y -- and "scaling" and "translation" just mean literally multiplying and adding, respectively.) you certainly do not need a 3d library for that, or a library of any sort. Zooming in and out in 2d vector graphics amounts to multiplying and adding.
Edit: ok, zooming in/out may also involve clipping. Liang-barsky clipping is pretty trivial though
Here's my implementation in C.