Added IECoreGL::ToGLStateConverter.

This converts a CompoundObject representing a whole block of attributes directly into an IECoreGL::State object. Its current intended use is in Gaffer, where it will allow us to implement a faster Gaffer->GLScene coupling. This does duplicate some functionality from IECoreGL::Renderer, but we're justifying it by the Gaffer refactor being an important step towards a GL scene graph without an awkward Renderer interface in the way.