let mouse clicks change active photo

Program.cs

@@ -245,6 +245,17 @@ public class Game : GameWindow {
 
     KeyboardState input = KeyboardState;
 
+    if (MouseState.IsButtonPressed(0)) {
+      for (int i = 0; i < geometry.ThumbnailBoxes.Count; i++) {
+        Box2i box = geometry.ThumbnailBoxes[i];
+        if (box.ContainsInclusive((Vector2i) MouseState.Position)) {
+          if (0 <= i && i < textures.Count) {
+            textureIndex = i;
+          }
+        }
+      }
+    }
+
     if (input.IsKeyDown(Keys.Escape)) {
       Close();
     }