Fix converting v3 mesh normals and tangents

[nikitalita]

What problem(s) does this PR solve?

• Closes ArrayMeshes converted from Godot 3.x display incorrectly in 4.0 #63550

Additional information

There were a number of issues in the functionality to convert the old ArrayMesh format to the new one:

• The Tangent array data was accidentally written to the Normal array data as a result of an accidental use of the ARRAY_NORMAL offset
  • As a result of this, the mesh was loaded with garbage normals and garbage tangents
• Compressed normals and tangents weren't being properly decompressed before attempting to encode them
  • when compressing normals and tangents without octahedral compression, they were multiplied by 127 and stored as an uint8_t (see the old code); however, we were not dividing them by 127 when reading them.
• The old value of ARRAY_FLAG_USE_2D_VERTICES was set on the new ArrayMesh format instead of the new one