Creates a new user font-face.
Use the setter functions to associate callbacks with the returned
user font. The only mandatory callback is render_glyph.
After the font-face is created, the user can attach arbitrary data
(the actual font data) to it using FontFace.setUserData
and access it from the user-font callbacks by using
ScaledFont.getFontFace followed by
FontFace.getUserData.
Since 1.8
Creates a new user font-face. Use the setter functions to associate callbacks with the returned user font. The only mandatory callback is render_glyph. After the font-face is created, the user can attach arbitrary data (the actual font data) to it using FontFace.setUserData and access it from the user-font callbacks by using ScaledFont.getFontFace followed by FontFace.getUserData. Since 1.8