Add stub demo-playback code

src/Ecs/Lockstep.ts

@@ -80,6 +80,24 @@ export class LockstepState<Input, State> {
     }
 }
 
+export class Playback<Input, State> {
+    private frame = 0;
+
+    public constructor(
+        private state: State,
+        private inputLog: Input[],
+        private engine: LockstepProcessor<Input, State>,
+    ) {}
+
+    public getNextState(): State {
+        if(this.frame < this.inputLog.length) {
+            this.engine.advanceState(this.state, this.inputLog[this.frame]);
+            this.frame++;
+        }
+        return this.state;
+    }
+}
+
 export class LockstepLoop<Input, State extends DeepCopy<State>> {
 
 }