Use flextGL OpenGLRaw bindings instead of OpenGLRaw bindings.

There is still a dependency to OpenGLRaw because we make use of the
"getProcAddress" function. However, all the bindings themselves now come from
an automatically generated Haskell file. <https://github.com/Noeda/flextGL>

This way we avoid hacks to bring some raw bindings that OpenGLRaw did not have.