Add Gears example

I also changed glGetString to return a `<c-string>`. We can't map `<GLubyte*>` to `<c-string>` because `<GLubyte*>` is used as an input for *ubv functions.

Converting a sequence of floats (e.g. `let s = #[0.0, 0.2, 0.5, 1.0]`) to `<c-float*>` with `as(<c-float*>, s)` doesn't work.