Fix concurrent issue of ASTC context

[isHarryh]

Summary

This PR mainly fixes the concurrent issue of ASTC context in Texture2DConverter. By the way, it also fixes some type errors and type hints in Texture2DConverter.

1. ASTC Context Concurrent Issue

How do I discovered this issue:

After upgrading UnityPy (from 1.18 to 1.20) in my project ArkUnpacker, I noticed that, when using multithreading to extract images, the application has an unpredictable chance to crash without any stacktrace (just like you shutdown the application in the Task Manager forcibly).

With the extensive debugging, I finally determined that it is the asynchronous access to the ASTC context that causes the problem.

This issue was caused by this commit d38b5f6 which introduced astc_encoder for texture decoding.

How to fix it:

Simply adding a threading.Lock to the dict ASTC_CONTEXTS.

2. Type Errors

There are two potential type errors. See my commit for detailed changes. Here comes the explanation:

1. In this case, texture_format may be an integer.

        raise NotImplementedError(
-            f"Not implemented texture format: {texture_format.name}"
+            f"Not implemented texture format: {texture_format}"
        )

2. In this case, bytes cannot be divided by integer.

    rgb_data = b"".join(
        stream.read(padding_size * 2) + padding
-        for _ in range(image_data / (2 * padding_size))
+        for _ in range(len(image_data) // (2 * padding_size))
    )

[K0lb3]

Good catch, I forgot that texture_format could still be an int.
As the int number isn't that telling imo, it would be better to cast the format to the enum and print it's name if possible, and only show the number if that is not possible.

[isHarryh]

If the int can be cast to the enum, it is very likely to be a known format that has been implemented. In other words, the unimplemented texture format is very likely to be not castable. So showing the int value is enough.

[K0lb3]

Not all of them are implemented atm, but so far I haven't seen any of those in the wild either.
So I guess it's indeed fine to keep it as is as int.

[K0lb3]

Thanks for the PR and the fixes.
I just want to see two little changes overall, reducing the lock scope to the specific context usage and replacing Union[foo, None] with Optional[foo].

[K0lb3]

Thanks a lot 👍