TODO_QT5_GUI.TXT

new tile display logic
- fix offset
- fix position marker & route here marker
- check POI markers & routing overlay
-> routing overlay seems OK
- load tiles during drag if cornerX/cornerY changes changes

- tile instance recycling ? - DONE
-> eq, reset rather than destroy ? ;-)
 1) generate list of what should be on screen before and now, new_list & old_list
 2) generate a list of new_items that contains only items that are in
    new_list but not in old_list
 3) iterate over tilesModel:
    -> if tile is in new_list, do nothing
    -> if tile not in new_list:
     -> pop an item from new_items and set x & y accordingly
     -> if new_items is empty then remove the tile from the tilesModel
 4) iterate over new_tiles (if any) and add new tiles to the tilesModel
    accordingly

- areTilesAvailable() should send a batch notifications for all tiles once it finishes iterating
  over tile_ids; true for tiles that are available, false for the others - DONE

- idea: seamless zoom - use scaling of previous zoom level to "mask" zooming
 0) only trigger real zooming once user stops attempting to zoom in/out
 1) keep the tiles that will still be visible after being scaled appropriately
    visible and add tiles corresponding to the new zoom level
 2) remove the tiles from the previous zoom level once the new tiles cover the tile
    or possibly after a timeout