Sets the glyph rendering function of a user-font.
See cairo_user_scaled_font_render_glyph_func_t for details of how the callback
works.
The font-face should not be immutable or a CAIRO_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
Sets the glyph rendering function of a user-font. See cairo_user_scaled_font_render_glyph_func_t for details of how the callback works. The font-face should not be immutable or a CAIRO_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