letterbox the image so that the aspect ratio is right

Program.cs

@@ -278,13 +278,18 @@ public class Game : GameWindow {
     int thumbnailWidth = 150;
     int thumbnailHeight = 100;
     int borderWidth = 2;
-    int photoWidth = windowWidth - thumbnailWidth - borderWidth;
+    int maxPhotoWidth = windowWidth - thumbnailWidth - borderWidth;
 
-    SetVertices(0, 0, photoWidth, windowHeight);
+    Texture active = textures[textureIndex];
+    // TODO: handle the case where we need to letterbox vertically instead.
+    int photoWidth = (int) (1.0 * windowHeight / active.Height * active.Width);
+    int letterboxWidth = (maxPhotoWidth - photoWidth) / 2;
+
+    SetVertices(letterboxWidth, 0, photoWidth + letterboxWidth, windowHeight);
     GL.BindBuffer(BufferTarget.ArrayBuffer, VertexBufferObject);
     GL.BufferData(BufferTarget.ArrayBuffer, vertices.Length * sizeof(float), vertices, BufferUsageHint.DynamicDraw);
     GL.ActiveTexture(TextureUnit.Texture0);
-    GL.BindTexture(TextureTarget.Texture2D, textures[textureIndex].Handle);
+    GL.BindTexture(TextureTarget.Texture2D, active.Handle);
     shader.Use();
     GL.DrawElements(PrimitiveType.Triangles, indices.Length, DrawElementsType.UnsignedInt, 0);