Fix race condition between inbound and injected packets (fixes #118)

In the case of #118, this was resulting in the EntityTrackerUpdate packet (which
contains info on which skin layers should be visible) sometimes ending up before
the SpawnPlayer packet, hence the players outer skin layers being invisible at
the start of the recording.

src/main/java/com/replaymod/core/versions/MCVer.java

@@ -394,6 +394,14 @@ public class MCVer {
         //#endif
     }
 
+    public static void scheduleOnMainThread(Runnable runnable) {
+        //#if MC>=11400
+        getMinecraft().send(runnable);
+        //#else
+        //$$ getMinecraft().addScheduledTask(runnable);
+        //#endif
+    }
+
     //#if MC>=11400
     public static Window getWindow(MinecraftClient mc) {
         //#if MC>=11500

src/main/java/com/replaymod/recording/mixin/MixinNetHandlerPlayClient.java

@@ -57,6 +57,7 @@ public abstract class MixinNetHandlerPlayClient {
     //$$ @Inject(method = "handlePlayerListItem", at=@At("HEAD"))
     //#endif
     public void recordOwnJoin(PlayerListS2CPacket packet, CallbackInfo ci) {
+        if (!MCVer.isOnMainThread()) return;
         if (mcStatic.player == null) return;
 
         RecordingEventHandler handler = getRecordingEventHandler();

src/main/java/com/replaymod/recording/packet/PacketListener.java

@@ -149,6 +149,14 @@ public class PacketListener extends ChannelInboundHandlerAdapter {
     }
 
     public void save(Packet packet) {
+        // If we're not on the main thread (i.e. we're on the netty thread), then we need to schedule the saving
+        // to happen on the main thread so we can guarantee correct ordering of inbound and inject packets.
+        // Otherwise, injected packets may end up further down the packet stream than they were supposed to and other
+        // inbound packets which may rely on the injected packet would behave incorrectly when played back.
+        if (!MCVer.isOnMainThread()) {
+            MCVer.scheduleOnMainThread(() -> save(packet));
+            return;
+        }
         try {
             if(packet instanceof PlayerSpawnS2CPacket) {
                 //#if MC>=10800