Fix reading manifest in F3D importer

[galou]

Before, importing failed with the manifest I had.

The length of the unknown array a1 was incorrectly assumed to be 2 whereas it was 4 in my case. The length is now taken from the distance to the next known field Application.

There's commit bcbd2c2a54bc that also deals with the manifest but the changes there after reading cnt, so that the bug should also affect it.

box_v4.zip is the Fusion 360 file I received from someone.