Add a layer for OpenGL version 3

OpenGL has undergone significant changes since version 1.1, for which IceT was originally designed. It is no longer a fixed pipeline, and the API is much more buffer-centric. That is, it is common to create buffers as targets for rendering.

To start taking advantage of new features, this is introducing a new render layer for OpenGL 3 (and beyond). It has a new callback type. Rather than expect all of the state set in the OpenGL fixed-function pipeline, parameters are passed much like the generic callback. The current form of a callback establishes 2 OpenGL texture buffers that the callback is expected to fill.