Fix call queue problems when loading TileSet

[KoBeWi]

When TileSet is being loaded, it will load every TileSetAtlasSource and it will emit changed signal for every tile and tile property. The signal is connected to _queue_update_padded_texture(), which doesn't have any safeguard against multiple calls, which means that it did a deferred call every time. So for my tileset with 40+ sources, 50+ tiles each that's like... 2000 deferred calls.

The fix in this PR is twofold:

• it adds a safeguard in _queue_update_padded_texture() to prevent multiple deferred calls
• it prevents redundant changed emissions while the TileSet is being loaded

Fixes the memory problem in #87593, effectively resolving the issue, as the project can load (albeit it's still extremely slow). The problem was that the deferred calls would build up and never get flushed, because many scenes were loaded one after another and the TileSet was repeatedly freed and loaded again (which is also why it takes so long). I have ~500 scenes that use this TileSet, so if you do the maths, it's obvious why the queue couldn't hold it.

Unfortunately it still results in some deferred calls that will very slowly clog up message queue, but since this makes like 100x less of them, it's much less likely to run out of memory. Not sure how to prevent it completely; we could maybe remove deferred call from queue manually by the TileSetAtlasSource if it's being deleted, but MessageQueue has no method for that AFAIK.

[AThousandShips]

LGTM, makes sense

[akien-mga]

Thanks!