Provide a way to bind the current framebuffer.

[InquisitiveCoder]

Libretro requires calling glBindFramebuffer every frame. I couldn't find a way to do this through the golem API. Since golem takes ownership of the glow Context, I'm currently working around this by creating two glow::Context objects; one for calling bind_framebuffer and another for golem.

It'd be handy if I could borrow a reference to the glow Context, even if I have to use an unsafe method.

[elliegoldstein]

I don't really work on Golem anymore (as you can probably tell), but if you want to put a PR together I'd take a look.