merged develop and fixed the very broken demos

Source/MonoGame.Extended.Entities/Systems/DrawSystem.cs

@@ -0,0 +1,16 @@
+using Microsoft.Xna.Framework;
+
+namespace MonoGame.Extended.Entities.Systems
+{
+    public interface IDrawSystem : ISystem
+    {
+        void Draw(GameTime gameTime);
+    }
+
+    public abstract class DrawSystem : IDrawSystem
+    {
+        public virtual void Dispose() { }
+        public virtual void Initialize(World world) { }
+        public abstract void Draw(GameTime gameTime);
+    }
+}

Source/MonoGame.Extended.Entities/Systems/EntitySystem.cs

@@ -46,5 +46,6 @@ namespace MonoGame.Extended.Entities.Systems
 
         protected void DestroyEntity(int entityId) => _world.DestroyEntity(entityId);
         protected Entity CreateEntity() => _world.CreateEntity();
+        protected Entity GetEntity(int entityId) => _world.GetEntity(entityId);
     }
 }

Source/MonoGame.Extended.Entities/Systems/IDrawSystem.cs

@@ -1,9 +0,0 @@
-using Microsoft.Xna.Framework;
-
-namespace MonoGame.Extended.Entities.Systems
-{
-    public interface IDrawSystem : ISystem
-    {
-        void Draw(GameTime gameTime);
-    }
-}

Source/MonoGame.Extended.Entities/Systems/IUpdateSystem.cs

@@ -1,9 +0,0 @@
-using Microsoft.Xna.Framework;
-
-namespace MonoGame.Extended.Entities.Systems
-{
-    public interface IUpdateSystem : ISystem
-    {
-        void Update(GameTime gameTime);
-    }
-}

Source/MonoGame.Extended.Entities/Systems/UpdateSystem.cs

@@ -0,0 +1,16 @@
+using Microsoft.Xna.Framework;
+
+namespace MonoGame.Extended.Entities.Systems
+{
+    public interface IUpdateSystem : ISystem
+    {
+        void Update(GameTime gameTime);
+    }
+
+    public abstract class UpdateSystem : IUpdateSystem
+    {
+        public virtual void Dispose() { }
+        public virtual void Initialize(World world) { }
+        public abstract void Update(GameTime gameTime);
+    }
+}