I agree 100% with Tim that using OpenGL directly as a rendering API for GTK+ makes no sense. The OpenGL 2D API is outdated and irrelevant; using the 3D API for acceleration of 2D is possible, but involves a lot of deep hackery; you want something like glitz to hide the details.
But does that mean there is no use for 3D rendering engines in the GTK+ world? I don't think that's the case. One idea I've been thinking about recently is that a 3D engine could provide a way of doing flexible theming in a way that is more accomodating of custom widgets than pixmap-based approaches. With a pixmap-based approach, if you know how to theme a button, that doesn't tell you how to to theme something that is somewhat like a button. For example, imagine that you had a button separated into two parts with a shallow groove in between, similar to what you often see for a drop-down on a browser's back button.
A theme that knows how to draw a button won't know how to draw a button with a groove. But if the custom widget calls the theme API not as "draw a button", but as "draw a button with this skeleton", where the skeleton is a two-and-a-half dimension shape like:
![](http://fishsoup.net/blog_images/widget-skeleton.png)
Then the theme can apply textures, pixel shaders, and possibly transformations to the shape to get the final appearance. The same approach that produces a button from the basic box shape should produce something reasonable from the custom skeleton.
![](http://fishsoup.net/blog_images/widget-realized.png)
(A poor quality mockup in the GIMP.) Note that with pixel shaders, you aren't restricted to pure 3D looks; it should be possible to get a flat high-contrast appearances if you want that. Another possible point of control for the theme engine would be modifying the skeleton a bit by, say, rounding off the corners.
This approach should allow a very wide range of themes; what it doesn't allow easily is themes that reproduce anything that a graphics designer could possibly dream up and draw in the GIMP or Photoshop. So, it would be very important to have interactive user-friendly tools for theming that allow the graphics designer to play around, get a sense of the possibilities and create themes without having to go through the intermediatary of a programmer.