feat: add LoadingScreen with full-screen triangle shader

Assets/shaders/loading.frag

@@ -0,0 +1,3 @@
+#version 330 core
+out vec4 FragColor;
+void main() { FragColor = vec4(0.10, 0.12, 0.16, 1.0); }

Assets/shaders/loading.vert

@@ -0,0 +1,3 @@
+#version 330 core
+layout(location = 0) in vec2 aPos;
+void main() { gl_Position = vec4(aPos, 0.0, 1.0); }

Rendering/LoadingScreen.cs

@@ -0,0 +1,52 @@
+using OpenTK.Graphics.OpenGL4;
+
+namespace VoxelEngine.Rendering;
+
+public sealed class LoadingScreen : IDisposable
+{
+    private readonly int _vao;
+    private readonly int _vbo;
+    private readonly Shader _shader;
+
+    public LoadingScreen()
+    {
+        this._shader = new Shader("Assets/shaders/loading.vert", "Assets/shaders/loading.frag");
+
+        // Single triangle covering the entire NDC quad.
+        float[] vertices =
+        {
+            -1f, -1f,
+             3f, -1f,
+            -1f,  3f,
+        };
+
+        this._vao = GL.GenVertexArray();
+        GL.BindVertexArray(this._vao);
+
+        this._vbo = GL.GenBuffer();
+        GL.BindBuffer(BufferTarget.ArrayBuffer, this._vbo);
+        GL.BufferData(BufferTarget.ArrayBuffer, vertices.Length * sizeof(float), vertices, BufferUsageHint.StaticDraw);
+
+        GL.VertexAttribPointer(0, 2, VertexAttribPointerType.Float, false, 2 * sizeof(float), 0);
+        GL.EnableVertexAttribArray(0);
+
+        GL.BindVertexArray(0);
+    }
+
+    public void Draw()
+    {
+        GL.Disable(EnableCap.DepthTest);
+        this._shader.Use();
+        GL.BindVertexArray(this._vao);
+        GL.DrawArrays(PrimitiveType.Triangles, 0, 3);
+        GL.BindVertexArray(0);
+        GL.Enable(EnableCap.DepthTest);
+    }
+
+    public void Dispose()
+    {
+        GL.DeleteBuffer(this._vbo);
+        GL.DeleteVertexArray(this._vao);
+        this._shader.Dispose();
+    }
+}