code: func (v *Context) FillExtents() (x1, y1, x2, y2 float64) {
  	var cx1, cy1, cx2, cy2 C.double
  	C.cairo_fill_extents(v.native(), &cx1, &cy1, &cx2, &cy2)
  	return float64(cx1), float64(cy1), float64(cx2), float64(cy2)
  }
docs: // FillExtents is a wrapper around cairo_fill_extents().