Add Color record struct, small refactor, create a DaggerFramework.SceneGraph namespace.

Source/SceneGraph/Entities/Entity2d.cs

@@ -0,0 +1,9 @@
+using System.Numerics;
+
+namespace DaggerFramework.SceneGraph
+{
+    public class Entity2d : Entity
+    {
+        public Vector2 Position { get; set; }
+    }
+}