doc: opengl: document the OpenGL pipeline
Provides documentation on the OpenGL pipeline, with details and examples on how to extend the current pipeline through modules.
Co-authored-by: Simon Latapie garf@videolabs.io
The documentation in itself is not finished, it's missing parts, context and graphs in particular, but it starts to show how it's planned to be integrated.
I'd also like to integrate it better into the doxygen, but I'm not sure how yet.