Switch TagOverlay to use river_shell_surface_v1

This follows the same patterns that Wallpaper and Bar did and makes
TagOverlay use the same manage/render cycle as the rest of the WM.

We also switched to just use a poll timer like river-tag-overlay instead
of using the timerfd. I realized that the Zig stdlib doesn't actually
support timerfds for FreeBSD right now and I don't feel like adding them.

src/Buffer.zig

@@ -161,5 +161,10 @@ const seal = switch (builtin.target.os.tag) {
     },
     else => @compileError("target OS not supported"),
 };
+comptime {
+    if (@hasField(os.linux.F, "SEAL_SEAL")) {
+        @compileError("SEAL_SEAL added to std.os.linux, get rid of the hardcoded values above.");
+    }
+}
 
 const log = std.log.scoped(.Buffer);