Offscreen plotting in OpenGL

[Jordi Gutiérrez Hermoso]

So there's one outstanding wishlist item for making the OpenGL backend
default for 3.6, and that's offscreen plotting.

I asked around, and apparently it's now easier to do this than it was
3 years ago, and framebuffer objects are the way to go. It doesn't
sound terribly complicated from my initial perusal of the subject.
Should we "officially" (whatever that means) add this to the 3.6
release goals?

- Jordi G. H.