Grzegorz Owsiany

[C++] Vtable deduplication fails for 64-bit buffers >2GB, causing slow serialization and larger buffers

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### Description When serializing FlatBuffers with 64-bit offsets (for buffers larger than 2GB), vtable deduplication fails due to incorrect offset calculations. This results in significantly slower serialization times and larger-than-expected...

Fixes #8590 ### Summary This PR corrects vtable deduplication for FlatBuffers using 64-bit offsets. Previously, when serializing buffers larger than 2GB, vtable deduplication failed, resulting in much slower serialization and...