Core: Fix packBits heap-buffer-overflow

[zach2good]

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• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Godbolt link: https://godbolt.org/z/abn84e8PT (uncomment Original:: functions to see ASan lose it's shit)

==11932==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6020000000f1 at pc 0x00010025c9a0 bp 0x00016fba1e30 sp 0x00016fba1e28
READ of size 4 at 0x6020000000f1 thread T0
    #0 0x00010025c99c in Original::packBitsBE(...) repro_packbits.cpp:32
    #1 0x00010025cc60 in Original::packBitsLE(...) repro_packbits.cpp:51

0x6020000000f4 is located 0 bytes after 4-byte region [0x6020000000f0,0x6020000000f4)
allocated by thread T0 here:
    #1 0x00010025cb20 in Original::packBitsLE(...) repro_packbits.cpp:46

In packBitsLE, the bytesNeeded logic often calculated a larger buffer (e.g., 4 bytes for 17 bits) and copied the entire buffer back to the target, even if the bitfield only spanned 3 bytes.

Fix:

• Copy everything into local buffers of the relevant data width before writing them back later: no opportunity for overflow
• Don't use this: if ((lengthInBit + bitOffset) <= 8)

Testing:
/sea all still works