cairoUserFontFaceSetRenderGlyphFunc(font.face, render.glyph.func)CairoFontFace] A user font faceNULLCAIRO_STATUS_USER_FONT_IMMUTABLE
error will occur. A user font-face is immutable as soon as a scaled-font
is created from it.
The render_glyph callback is the only mandatory callback of a user-font.
If the callback is NULL and a glyph is tried to be rendered using
font.face, a CAIRO_STATUS_USER_FONT_ERROR will occur.
Since 1.8