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

TestGame/Circle2d.cs

@@ -1,12 +1,11 @@
 using DaggerFramework.Rendering;
-using System.Drawing;
-
+using DaggerFramework.SceneGraph;
 
 namespace DaggerFramework;
 
 public class Circle2d : Drawable2d
 {
-    public override void OnDraw(in Renderer renderer)
+    public override void OnDraw(Renderer renderer)
     {
         renderer.DrawCircle(32f, Color.AliceBlue);
     }