[ShellGtkEmbed] base this on ClutterX11TexturePixmap, not ClutterGLX

As of 1.4, ClutterGLXTexturePixmap is identical to
ClutterX11TexturePixmap (with the differences having been moved into
the cogl layer). So make ShellGtkEmbed use the (introspected) X11
version, which then allows us to make the instance and class structs
public as well.

https://bugzilla.gnome.org/show_bug.cgi?id=608869