Function gl::load_with

pub fn load_with<F>(loadfn: F) where
    F: FnMut(&'static str) -> *const c_void, 

Load each OpenGL symbol using a custom load function. This allows for the use of functions like glfwGetProcAddress or SDL_GL_GetProcAddress.

gl::load_with(|s| glfw.get_proc_address(s));