This is now possible to do in FontRenderer, but needs implementation on a per overlay basis.

This is a tricky one. Are we sure we want to do that? I seriously think that "font scaling" is a separate UI concern from "outline scaling" of overlays.

To be clear: I agree that it should be possible to scale fonts. But it should not be part of rescaling the size of overlays. We will need a separate way to change font sizes. Perhaps using the scroll wheel in the overlay editor? Or a slider or other widget.

Technically, this is fixed, where it is needed. #654 will track this.