Feature request: creating Surface from externally-created FBO (e.g. by libretro-frontend)

[Des-Nerger]

I wanna be able to use Glium when implementing libretro-based game engines. Within libretro, a game engine doesn't own the whole OpenGL context, all it has instead is a frontend-created FBO id, to be used through a glBindFramebuffer call. Details: https://github.com/libretro/libretro.github.com/raw/master/documents/libretro-gl.pdf

What's the easiest way to construct a glium::Surface from an existing, externally-created FBO?