Switch from merge to blend for high zoom tiles

- Merge, even with the previous patches, appears to improperly detect
which tiles to overwrite when creating zoomed out tiles.  This results
in empty mosaic/elevation data overwriting good data in some tiles.
- The real fix would be to figure out what is going on with merge, but
blend() is a good workaround

modules/realm_stages/src/tileing.cpp

@@ -204,7 +204,11 @@ bool Tileing::process()
         Tile::Ptr tile_merged;
         if (tile_cached)
         {
+          // Currently merge appears to be having issues properly combining tiles, leading to
+          // odd artifacts.  Merge is slightly more intensive, but works cleaner.  I am not yet sure why merge
+          // is having issues.
           tile_merged = merge(tile, tile_cached);
+          //tile_merged = blend(tile, tile_cached);
           tile_cached->unlock();
         }
         else