[cairo] [rcairo] a problem with "group_target" on SVGSurface

[Binzo]

Hello,

I got a error message using "group_target" on SVGSurface.

    `group_target': unknown source type: 4096 (ArgumentError)

The test script is here.
---
require "cairo"

s = Cairo::SVGSurface.new("output.svg", 200, 200)
c = Cairo::Context.new(s)

c.push_group do
  p c.group_target    ### exception raised here
end
---

When I used PDFSurface instead of SVGSurface, no exception was raised and
returned

   #<Cairo::ImageSurface:0x6ff824>

I use cairo-1.4.12 (via MacPorts) and rcairo-1.5.0 (via RubyGems) and
ruby 1.8.6 (2007-12-19 patchlevel 5000) [i686-darwin8.11.1].

Thanks.