FBX: Fix fbx2json tool for 7500 FBX files (FBX2016).

Based on patch from T49822, thanks a bunch for investigation.

Change is actually pretty limited - 7500 now uses some uint64 instead of
uint32 integers as some root element info (like size etc.), and
accordingly raised the 'stop nested data' marker from 13 to 25 NULL
bytes.

Next step: check if we can make actual importer load FBX2016 files that
simply!