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

Source/SceneGraph/Entities/CircleShape2d.cs

@@ -0,0 +1,18 @@
+using DaggerFramework.Rendering;
+
+namespace DaggerFramework.SceneGraph
+{
+    public class CircleShape2d : Drawable2d
+    {
+        public float Radius { get => _radius; set => _radius = value; }
+        public Color Color { get => _color; set => _color = value; }
+
+        public override void OnDraw(Renderer renderer)
+        {
+            renderer.DrawCircle(_radius, _color);
+        }
+
+        private float _radius;
+        private Color _color;
+    }
+}