This will fix import erros, when SubMeshes are not sharing the exact same attributes. e.g.: SubMesh 0 uses only Position and UV0, but SubMesh 1 would also using an extra UV layout.
Previously we checked only the first SubMesh, which attributes are required to build a Unity Mesh. This leads to uninitialized arrrays. Now we loop through all SubMeshes and create first a collection of all used Attributes, so we know which attribute arrays we need to create.

This fixed issue #808
Test File from the issue:
File