Provide a way to nuke glium's internal state

[HeroesGrave]

I'm trying to use glium with nanovg but trying to "reset" the state after calling nanovg's opengl renderer is rather difficult because I can't work out what needs to be reset, or what it need to be reset to. At the moment I'm digging through source code of both libraries to try and work it out, but this isn't ideal.

There should be a method that does the following:

1. Unbind everything
2. Take a DrawParameters struct and set all the associated OpenGL state without checking the internal cache.
3. Override the internal cache with the new state.