perf(client): read the loot tables while joining a server

On a server's world the loot tables were first read on the game thread by
the first block placed (StepContext's loot), a ~0.6 s freeze on a phone.
They are now read on a helper thread as the connection starts, like the
world screens' preload does for local worlds, and kept from then on.

crates/horizon-client/src/game/remote.rs

@@ -302,6 +302,7 @@ impl Game<'_> {
         name: &str,
         cookies: Option<horizon_client::net::Cookies>,
     ) -> Result<()> {
+        super::world_loading::preload_server_loot(&self.options.resources);
         if self.defer_world_entry(super::PendingWorldEntry::Server {
             address: address.to_owned(),
             name: name.to_owned(),

crates/horizon-client/src/game/world_loading.rs

@@ -279,6 +279,27 @@ pub(super) fn world_data_used() {
     *PRELOAD.lock().unwrap_or_else(|e| e.into_inner()) = Preload::Used;
 }
 
+/// A server's world opens: the loot tables its first placed block or broken
+/// container needs (`loot_data`) are read on a helper thread while the
+/// connection is made, so the game thread never waits on them (~0.6 s on a
+/// phone). Kept from then on, as a local world keeps them.
+pub(super) fn preload_server_loot(resources: &std::path::Path) {
+    world_data_used();
+    if horizon_client::structure_blocks::loaded_loot_data().is_some() {
+        return;
+    }
+    let resources = resources.to_owned();
+    let spawned = std::thread::Builder::new()
+        .name("Loot preload".into())
+        .spawn(move || {
+            horizon_threads::behind_server();
+            let _ = horizon_client::structure_blocks::loot_data(&resources);
+        });
+    if let Err(e) = spawned {
+        log::warn!("Horizon: loot preload: {e}");
+    }
+}
+
 pub(super) fn make_streaming(
     res: &Resources,
     pack: &PackResources,