added barebones ui rendering

UI/UiVertex.cs

@@ -0,0 +1,15 @@
+using OpenTK.Mathematics;
+
+struct UiVertex
+{
+    public Vector2 Position;
+    public Vector2 TexCoord;
+    public Vector4 Color;
+
+    public UiVertex(float x, float y, float u, float v, Vector4 color)
+    {
+        Position = new Vector2(x, y);
+        TexCoord = new Vector2(u, v);
+        Color = color;
+    }
+}