[Wayland] Add support for reconnects when the compositor restarts

[flibitijibibo]

For the most part this is a compositor thing, but SDL does play a small role in this. I discovered a mention of us while looking at Plasma's Wayland showstoppers:

https://community.kde.org/Plasma/Wayland_Showstoppers

https://invent.kde.org/plasma/kwin/-/wikis/Restarting

Since it looks like some kind of changes need to be made in SDL, we should have it in our records, so here it is. There's a patchset here but it's pretty old...

https://github.com/davidedmundson/SDL/commits/reconnect_mouse

... we should coordinate with them as work on compositor restarts continues. I'm still stuck in other work right now so this is not on my TODO at all.

[flibitijibibo]

Took another look at Edmunson's patch and it pretty much is all stuff that would be good to have, even before reconnects are supported by libwayland - we should clean up the patch for upstreaming and just comment the reconnect stuff with:

#if 0 /* TODO: Uncomment when https://invent.kde.org/plasma/kwin/-/wikis/Restarting is complete */

That way it's easy to flip the switch for both 2.x and 3.0, since that external stuff will probably happen after we branch.

[flibitijibibo]

Going to try and refresh this over the weekend so we have it in the 2.x series - not having this will be pretty annoying for cases where 20-compat isn't available.

[flibitijibibo]

Merged most of the patch: 9c8b1fd

The main thing that didn't get brought over was waylandmouse changes, which got blown apart when we starting using a different method to handle cursors. Shouldn't be too hard but it'll be "new" code rather than taking from an existing patch. If someone wants to beat me to this, you probably have a day or two to get a PR up!

[flibitijibibo]

Nevermind, turns out the new cursor work actually obsoleted a good chunk of it, so it was simpler than before: 571ff1a

That's all we can do until the new API is exposed, so back into 3.0 this goes!