Occasional crash when testing VoxelLodTerrain

[Zylann]

I'm sometimes getting a crash when testing VoxelLodTerrain. It's happening randomly, but so far it's been happening when duplicate_rejection is enabled in one of the threaded block managers. I need to investigate more to find the cause. It seems to happen more if you move fast in a very erratic way, back and forth etc.
I suspect this one has been there for a while according to 3465334

Clues:

Assigning an input block in shared input, but the previous value in the vector contains a Ref<T> with uninitialized memory, causing garbage delete. Considering stuff like this is able to run, it could be a vector overrun. I also found that in Godot debug builds, _DEBUG isn't defined on Windows, which disables STL boundary checks...

Inserting an item into the block_indexes HashTable which is used for duplicate rejection. But that hashmap's memory was already freed according to the magic number there

Okay, I think I found it.
duplicate rejection isn't locking the mutex to access block_indexes, and both threads are writing to it.

[Zylann]

Should be fixed by adc6a93