What is neccessary to implement a pixbuff-rendering backend?

[Benjamin <thronerbe gmx de>]

in cohesion to my previous post "Is it possible to render into memory on 
win32/gtk?" i think I've got a plan
that works without hacks, is portable and of general use for the gtk 
community:
   I would like to implement a pixbuf or pixmap (still not sure what 
the difference is) rendering backend
   for gtk. it will be similar to the Linux-fb backend, but more like 
the DirectFB backend (not because it will
   be hardware accelerated, but because it will have an own 
window-manager). The purpose of this backend
   will be the possibility to render gtk-widgets with all kinds of 
2D/3D APIs (OpenGL, DirectX, SDL,
   win32-GDI, what ever), that is supporting drawing of bitmaps (direct 
or as textures). Also i would implement
   methods that have to be called by the host-application to pass 
keyboard and mouse events to the Backend.  
   So GTK will be embedded into a wider range of Applications and will 
be of more general use for Full-Screen
   VR and Game-Applications that need to render their GUI with another 
API than with the ones already
   supported by GTK.

So i end up with some questions:

   A the moment i think it should be enough to implement the gdk 
runtime library. is it possible to tell GTK to use another
   GDK-runtime without recompile?

   what will by interface look like, what methods are called/required 
by GTK applications.

   will theme-engines still work? i think wimp will not work, because 
it is not using GDK for drawing (?) what is with others
   especially the pixmap/pixbuf theme engine?

Anything that points me into the right direction is welcome.

thanks in regard Benjamin - thronerbe[at]gmx.de