It would be good if Cairo consistently allowed passing NULL for any
out parameter to mean "ignore"

writing code like:

   double tx, ty;

   cairo_matrix_get_affine (scale,
                            &sc.matrix[0][0], &sc.matrix[0][1],
                            &sc.matrix[1][0], &sc.matrix[1][1],
                            tx, ty);

   return _ft_font_create (pattern, &sc);

Is painful for no good reason. This is most important for functions
with multiple out values. Though GTK+ applies the rule to *all* out
parameters, there is really no point for something like
cairo_text_extents().

cairo_matrix_get_affine()
cairo_current_rgb_color()
cairo_current_point()

Are the functions I can find that are affected.