Fix bug w/auto tileset mode adjusting tiles when we replace a unique instance of a tile+flag with a new tile

The assert() in remove_unused_tiles_from_tileset() checking for the
tiles histograms was failing because we weren't adjusting the
histogram correctly when a tile w/a flag is replaced w/another without
flags.

src/app/util/cel_ops.cpp

@@ -609,8 +609,9 @@ void modify_tilemap_cel_region(
 
         // We add the new one tileIndex in the histogram count.
         if (tilesetMode == TilesetMode::Auto &&
-            tileIndex != doc::notile &&
-            tileIndex >= 0 && tileIndex < tilesHistogram.size()) {
+            tile != doc::notile &&
+            tileIndex >= 0 && tileIndex < tilesHistogram.size() &&
+            ti != tileIndex) {
           ++tilesHistogram[tileIndex];
         }
       }