Gtk::GLArea: Fix refcount in signal_create_context()

gtk/src/glarea.hg: wrap() must take a reference on the GdkGLContext object
in signal_create_context().

Noticed by Bill McGrory <wdmcgrory@gmail.com> in
https://mail.gnome.org/archives/gtkmm-list/2016-April/msg00022.html