Drop the gtk-hint-font-metrics setting
With fractional scaling, we are much better off if we don't do any rounding of glyph positions or sizes during layout, since we don't know the device pixel grid and its alignment then, and we will round glyph positions to device pixels as appropriate during rendering.
This means that we want hint-metrics = off during layout. Unfortunately, we currently hardcode hint-metrics = on for unscaled rendering.