Do not generate delegates for overloads

We should only generate delegates for actual OpenGL entry points, not
for overloaded functions that resolve to the same entry point. This
improves loading speeds and reduces the size of the compiled dll.
Merge branch 'master' of https://github.com/thefiddler/opentk into modes
SDL_GL_GetAttribute requires a valid context

Calling SDL_GL_GetAttribute when context construction has failed leads
to erroneous behavior. This call should only be made when a context has
been constructed correctly.