Ignore mutex poisoning in X11_BACKEND

[notgull]

A panic doesn't really put any of the fields in XConnection into an invalid state, so there is no real reason to panic when poisoning is detected. So just ignore the poison.

Closes #3870

• [ ] Tested on all platforms changed
• [x] Added an entry to the changelog module if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented

[L3P3]

Any idea why this error could occur? Do I understand correctly that mutex poisoning might only happen if another thread panics? Why would we handle panics this way instead of handling the panic itself (I have not seen any other panic before)?

[kchibisov]

The thread is running outside of the winit itself, since winit doesn't spawn any threads at all, so where it happens is not our responsibility.