Freetype: Variable font named instances with slant not detected as oblique

When we add named instances to the font database, we manually check the axis for 'ital' to see if the instance is italic. However, there is also a 'slnt' axis which we are ignoring. At some slant, we should set the oblique style for this.

This is definitely an issue for the Freetype font database and for adding application fonts in Freetype, but it might also be an issue on macOS/Windows when adding application fonts. That needs to be checked.