Add constructor for existing Glutin window

[bvssvni]

Currently GlutinWindow can only be constructed from WindowSettings.

[adumbidiot]

I needed to do this so i created the following:

/// Creates a game window from a pre-existing Glutin event loop and window builder.
pub fn from_raw(settings: &WindowSettings, events_loop: glutin::EventsLoop, window_builder: glutin::WindowBuilder) -> Result<Self, Box<dyn Error>> {
    let title = settings.get_title();
    let exit_on_esc = settings.get_exit_on_esc();
    
    let context_builder = context_builder_from_settings(&settings)?;
    let ctx = context_builder.build_windowed(window_builder, &events_loop)?;
    let ctx = unsafe { ctx.make_current().map_err(|(_, err)| err)? };

    // Load the OpenGL function pointers.
    gl::load_with(|s| ctx.get_proc_address(s) as *const _);

    Ok(GlutinWindow {
        ctx,
        title,
        exit_on_esc,
        should_close: false,
        automatic_close: settings.get_automatic_close(),
        cursor_pos: None,
        is_capturing_cursor: false,
        last_cursor_pos: None,
        mouse_relative: None,
        events_loop,
        events: VecDeque::new(),
    })
}

I still pass in a WindowsSettings because we still need to pass in many settings from WindowsSettings in order to build a gl context and make a GlutinWindow. If this is acceptable I can open a pr.

[bvssvni]

@adumbidiot PR is welcome!

[bvssvni]

#185 does not technically construct from an existing Glutin window, but instead uses a window builder. Reopen if there is need for this edge case.