imgui: clarify the per-frame state setup a bit.
imgui: clarify the per-frame state setup a bit.

I'm putting a more descriptive version of this in the Context docs.

mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples