you can also try to use
ofEnableAntiAliasing();
ofEnableSmoothing();just before or even just after the fbo.begin() call.
Did something similar, had also crispy png exports from fbo, lots of trial & error to get it right.
some other things:
i was using settings.internalformat = GL_RGBA32F; and numsamples 8 or 16.
also, ofPAths have circle resolutions I think, setCircleResolution() & setCurveResolution(); try to use before adding the arc.
