* renderer_opengl: Namespace OpenGL code
Namespaces all OpenGL code under the OpenGL namespace.
Prevents polluting the global namespace and allows clear distinction
between other renderers' code in the future.
* Also namespace TextureCubeConfig
Internally these errors are handled by exceptions. Only fallbackable errors (that can be handled by CPU shader emulation) is reported. Completely ill-formed shader is still ASSERTed. Code logic related stuff is DEBUG_ASSERTed
