Until recently, Pango used FreeType to draw fonts. They will be moving over to Harfbuzz, an evolution of FreeType.
In short, FreeType was hard to work with. It required complex logic, and provided no advantage over Harfbuzz (other than being able to fetch opentype metrics with ease).
Upgrading to Pango v1.44 will break your GTK applications (if you use a
pcf bitmap font). Harfbuzz does support bitmap-only OpenType fonts,
otbs. Convert your existing fonts over to
FontForge. It is to be noted that applications
xft (X FreeType) to render fonts, and will
remain unaffected by the update.