the reference point is at the center of the surface.
the reference point is at the middle of the right edge.
the reference point is in the middle of the top edge.
the reference point is at the top right corner.
the reference point is at the top left corner.
the reference point is at the middle of the lower edge.
the reference point is at the lower right corner.
the reference point is at the lower left corner.
the reference point is at the top left corner of the surface itself, ignoring window manager decorations.
the reference point is at the middle of the left edge.
Defines the reference point of a surface and is used in
GdkPopupLayout.