Finish wiring up the TagOverlay

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).

docs/TODO.md

@@ -15,6 +15,7 @@ These are in rough order of my priority, though no promises I do them in this or
 - [ ] Save window positions between restarts
 - [ ] Support multiple seats
 - [ ] Support clipping floating windows on edge of/between outputs
+- [ ] Use per-output timerfds for tag overlay instead of a single shared one
 - [x] Support changeable primary ratio
 - [x] Support changeable primary count
 - [x] Support multiple outputs