Just like with Rust.
This Fixes the C++ opengl_texture example. Also improve advice for C++ OpenGL examples to explicitly select the FemtoVG GL renderer
Fixes#8901
Add the graphics state enum to the callback, too. When we add support for different backends,
it would be nice if it didn't require an API change.
It's duplicated from Rust
because it doesn't provide values. The WebLG one doesn't make sense for C++ and
the proc address closure isn't ffi safe.
(It could be manually bridged thought)