Make procaddr platform agnostic

In GoGL2 developers must include the correct procaddr package (wgl/darwin/etc.) for their platform otherwise gl.Init will fail because glt.GetProcAddress is nil. This design represents a regression from GoGL where the appropriate platform-specific GetProcAddress was defined automatically. Moreover GoGL2 does not signal an error in gl.Init if GetProcAddress is not set, instead the program panics.

Suggestions:
- Return an error if GetProcAddress is not set
- Use Go [build constraints](http://golang.org/pkg/go/build/) to automatically include the correct procaddr implementation