go through gps data backwards

Program.cs

@@ -704,16 +704,18 @@ public class Game : GameWindow {
 
     allPhotos.Sort(ComparePhotosByDate);
 
-    // Fix up photos with missing GPS.
+    // Fix up photos with missing GPS. We start at the end and work our way
+    // backwards, because if one photo is missing GPS, it's probably because
+    // the camera was turned off for a while, and whichever photo *after* it
+    // has GPS data is probably more accurate.
     GpsInfo? lastGps = null;
-    foreach (Photo p in allPhotos) {
-      if (p.Gps != null) {
-        lastGps = p.Gps;
-      }
+    for (int i = allPhotos.Count - 1; i >= 0; i--) {
+      Photo p = allPhotos[i];
       if (p.Gps == null) {
-        // FIXME: should we take from the photo immediately _before_, or after?
         Console.WriteLine("fixing GPS for " + p.Filename);
         p.Gps = lastGps;
+      } else {
+        lastGps = p.Gps;
       }
     }
     photos = allPhotos;