[Editor] Fix threading problems with TileMap preview

[AThousandShips]

As investigated in #87458 this is due to threading problems in the editor, specifically the preview generation, this solves this by handling deferred calls on a separate queue, rather than adding thread safety mechanisms to TileMapLayer which I'd say aren't necessary otherwise due to thread safety guarantees, I also suspect there are other possible race conditions here that won't be solved by adding those thread safety features, instead I suggest removing the thread safety from the equation entirely by controlling the timing

The general topic of thread safety of nodes not on the tree is a problem and other nodes suffer from problems due to deferred calls AFAIK, so a broader investigation of that and how TileMap and TileMapLayer relates to this should be done as well, but I think the pressing concern here is the editor related crash

• Fixes: Crash when editing/opening a huge tileset atlas #86226

Alternative to:

• Prevent tilemap editor hang and crash by mutex guarding tile_map_layer dirty.cell_list. #87458

[AThousandShips]

We could also handle this by not doing the internal update off the tree at all, but that is a solution that has various possible impacts so a more complicated thing

[RandomShaper]

Notwithstanding the idea of further investigation, and with the disclaimer that I haven't checked the details, this looks like the right thing to do.

[groud]

This might fix #86097 too.

[AThousandShips]

Sounds like a duplicate of #86226, but unable to verify right now

[YuriSizov]

Thanks!

Edit: Almost forgot to meme! Well, if @RandomShaper is happy, then we're happy.

[AThousandShips]

Thank you!

[YuriSizov]

Cherry-picked for 4.2.2.