Move photoIndex to wherever the previously active photo was

Program.cs

@@ -714,9 +714,22 @@ public class Game : GameWindow {
 
   void FilterByRating(int rating) {
     Console.WriteLine("filter to " + rating);
+    Photo previouslyActive = photos[photoIndex];
     photos = allPhotos.Where(p => p.Rating >= rating).ToList();
-    // TODO: put this closest to wherever the previously active photo was.
-    photoIndex = 0;
+    // Move photoIndex to wherever the previously active photo was, or if it
+    // was filtered out, to whichever unfiltered photo comes before it. This
+    // is O(n) in the length of allPhotos, but how bad can it be? :)
+    photoIndex = -1;
+    for (int i = 0; i < allPhotos.Count; i++) {
+      Photo candidate = allPhotos[i];
+      if (candidate.Rating >= rating) {
+        photoIndex++;
+      }
+      if (candidate == previouslyActive) {
+        break;
+      }
+    }
+    photoIndex = Math.Max(0, photoIndex);
   }
 
   protected override void OnLoad() {