Added unbuffered single thread Renderer

Closes https://github.com/gfx-rs/gfx-rs/issues/319

* Exposed `buf` to allow multi-thread rendering
* Added constructor on Renderer
* Implemented CommandBuffer for GlDevice
Making it public is a security concern. As long as it was hidden, the Renderer implementation could guarantee that the cached state is valid. Once you open it, there is no guarantee any more, thus making the architecture more shaky.