MissingOpenGLDefinitions within JUCE namespace (not portable without using namespace juce)

I’ve made some changes to the way that we load OpenGL symbols which should solve some of the issues raised in this thread.

Given that the change is quite large, it’s currently on a feature branch: generated-opengl-headers. I’d really appreciate any feedback on this potential change, before we merge it to develop. As always, I’m happy to answer any questions you may have about this change.

Some more details can be found in this post: FR: Wrangle a comprehensive set of OpenGL functions & macros - #5 by reuk