> Instead, what we need is a simple API with opaque handles and C function calls. No fancy OOP or macro preprocessors. Simple OpenGL style.
Something like the enlightenment foundation libraries might be what you want[1]. I haven't tried it out properly, but I liked the look of it from the docs, it's a C api and moderately minimalistic.
However, even with C APIs, all these things end up with something like "fancy OOP" (after all, even Gtk is a C Api). That's because GUI widgets seem to be one of the few places really well suited to OO ideas like inheritance and non-trivial getter-setters.