We had to fix a couple of compile errors that weren't showing while it
wasn't wired up (since I never just tried to compile TagOverlay.zig on
its own). We also changed the lifecycle to re-create/destroy the surface
to show/hide it, similar to the way that river-tag-overlay actually did
it.
Finally, I added @branchHint(.cold) to a few places in the event loop
where, if we're in the branch, the wm is definitely exiting, so it's
fine if they're cold (should almost never happen).
