Make generated bindings no_std compatible and add CStr symbol names feature

[nigrodev]

Generated bindings no_std compatibility

The generated bindings was already using core functions, but was getting them through re-exports of std. This is now replaced to use the original core ones. Now the generated bindings works in no_std enviroments 🎉

&CStr symbols support

Right now, if the user needs a null-terminated c-string from the gl::load_with function, they need to convert the &str to a CString. Since CString is only available in std and requires an allocator, the usage in std enviroments is limited.

With this pull request, using Rust 2021 edition C-string literals, the user have the possibility to enable the feature cstr_symbols on both gl_generator and gl to generate the bindings using &'static CStr instead of &'static str.

Examples of this problem can be seen on both SDL2 and SDL3 safe wrappers, where they need to perform a conversion using CString.

[Lokathor]

This looks really cool, but I think unfortunately this crate is effectively unmaintained for years now.

Last I heard the best crate to use for GL is glow.

[nigrodev]

Last I heard the best crate to use for GL is glow.

I've been using this crate because it's the only one that looks the most like raw OpenGL with C, glow is more "high level".

I recently read about some people being able to use Rust on consoles that don't support std, replacing the std calls with SDL. So I was experimenting with using the new SDL3 and opengl in a no_std environment with rust.

This looks really cool, but I think unfortunately this crate is effectively unmaintained for years now.

Since I've already made the changes, I decided to post it here anyway. At least, if anyone looks for it, they'll find it here :P

[Lokathor]

I had a similar concern about no_std, and so I made the gl33 and gl46 crates at one point, though I don't really get them updates either. Most people working on new graphics seem to want to work with Vulkan or WGPU, so I sorta lost interest in keeping up with the periodic minor updates to GL stuff.