Displaying framework has been barely workable

While I am not quite familiar with Gtk OpenGL extension, I would use Cairo surface with some Gtk functions to display image for the time being. And I decide not to change the mechanism in the near future, but it should be replaced by OpenGL surface after most of the core rendering function done.

Gtk with Cairo works OK for now despite the considerable delay because of the waiting of Gtk expose event. The rendering function is now running in a separated thread from the main thread where the GUI is run.

Although there are so many defects, everything seems OK for now :)