gtk2 test: add a test for `Gdk::Pixbuf.from_drawable`
gtk2 test: suppress a warning

    gtk2/test/test_gdk_pixbuf.rb: line 27
        Gdk-CRITICAL **:IA__gdk_pixbuf_get_from_drawable: assertion `gdk_window_is_viewable (src)' failed
gdk3: add `Gdk::Pixbuf.from_window`

TODO: suppress warnings in tests.
gdk3: remove deprecated methods
gdk3: remove a trailing space
gdk3: add `Gdk::Pixbuf.from_surface`
clutter: skip a not defined task

Because the following error occurred in my environment:

    % cd clutter
    % rake gem
    no configuration section for specified version of Ruby (rbconfig-i386-mingw32-2.1.1)
    rake aborted!
    Don't know how to build task 'native:clutter:i386-mingw32'
gdk3-gi: merge changes in gdk3
gdk3: rename to gdk3-no-gi
gdk3-gi: rename to gdk3
gdk3-no-gi: remove from build target
gdk3: split loader
glib2: fix free function
gdk3: support Gdk::Color.new
gdk3: support constants in Gdk::Event
gi: support instantiating union
gdk3: support Gdk::EventAny
gdk3: support Gdk::RGBA.new(red, green, blue, alpha)
glib2: clear by 0 for new boxed instance
gdk3: support WindowAttr