Thanks to Wayne Blaszczyk
Fixes https://gitlab.gnome.org/GNOME/vala/issues/1381
public Cairo.Filter get_filter ();
public Cairo.Status get_linear_points (out double x0, out double y0, out double x1, out double y1);
public void get_matrix (out Cairo.Matrix matrix);
+ public Cairo.Status get_rgba (out double red, out double green, out double blue, out double alpha);
public Cairo.Status get_surface (out unowned Cairo.Surface surface);
public Cairo.PatternType get_type ();
[CCode (simple_generics = true)]