Key/Button::is_pressed are not thread safe

[crumblingstatue]

At least on X11, it can trigger all kinds of errors to call them from a different thread.
And someone messaged me that it can cause segfaults for them to do so.

[memoryleak47]

One partial solution to this would to add a dummy &RenderWindow parameter to Key::is_pressed. This should prevent it from being called in another thread than where the RenderWindow lives, because RenderWindow is !Sync.

This wouldn't work whenever the user creates multiple RenderWindows though.

[crumblingstatue]

Looks like is_pressed isn't the only misbehaving API. There can be threading related errors for example if you create 2 sf::RenderWindows on 2 different threads. And there is probably more.

I don't have any clear plan on how to solve all of these threading issues, so for the time being I just added a warning to the documentation.

[memoryleak47]

What about adding a static unique_threadid: OnceCell<ThreadId> containing the thread id of the thread in which a RenderWindow is spawned.
And in RenderWindow::new() one

• sets unique_threadid to thread::current().id() if unique_threadid.get().is_none()
• does nothing if unique_threadid.get() == Some(thread::current().id())
• returns an error / panics, if the unique_threadid.get().is_some() but contains another thread-id than thread::current().id().

Using this, every RenderWindow has to live in the same thread.

This then allows to either:

• let non-thread-safe functions panic / error if they are called from the wrong thread (by comparing thread::current().id() to unique_threadid)
• add a dummy &RenderWindow argument to every non-thread safe function, which implies that this function is only called from the unique_threadid-thread.

[crumblingstatue]

That sounds like it might work. If you'd like you can submit a pull request, but for now I'll try to explore what other thread safety issues we might have.