Change surface to be Send

[vially]

As it was recently discussed on matrix, it turns out the GL Surface can also be made Send (similar to the Context, which is Send already).

Warning

I've only tested the example program on Wayland, so there's a good chance it might not work on other platforms (and I don't have an easy way to test it either).

Also, I'm just getting started with Rust, so the example code is definitely not very idiomatic, and most likely also incorrect, so please review it carefully. However, I don't mind iterating on it to get it in a better shape, if someone is willing to guide me in the right direction.

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

[kchibisov]

I'll look into example latter.

[kchibisov]

macOS tested.
X11 tested.

[kchibisov]

Windows tested.

[kchibisov]

Pushed 0.31.3 with that, let's see how it survives.