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