Crash on latest versions

[thesamet]

I was following the rotating square tutorial. It works well with the versions documented in the README.md. However, when trying to upgrade:

--- a/Cargo.toml
+++ b/Cargo.toml
-piston2d-graphics = "0.44.0"
-piston2d-opengl_graphics = "0.85.0"
+piston2d-graphics = "0.45.0"
+piston2d-opengl_graphics = "0.86.0"

I am getting

thread 'main' panicked at /home/thesamet/.cargo/registry/src/index.crates.io-6f17d22bba15001f/piston2d-opengl_graphics-0.86.0/src/back_end.rs:457:9:

    OpenGL function pointers must be loaded before creating the `Gl` backend!
    For more info, see the following issue on GitHub:
    https://github.com/PistonDevelopers/opengl_graphics/issues/103

Environment is NixOS and Wayland.

[DavideCanton]

I resolved by adding gl as a dependency and adding gl::load_with(|s| window.get_proc_address(s) as *const _);

I guess it should be done automatically in the future