Meshes with more than 65k triangles do not bake correctly with Progressive Lightmapper (Black artifacts)

1. Open the attached project 'progressive65k_mesh'
2. Open the scene 'highpoly_test'
3. Let the autobake bake the scene with the 63k mesh visible
4. Observe the scene bakes correctly
5. Hide the 63k mesh (scape4_63k)
6. Unhide the object 'scape4_73k'
7. Let the autobake run
8. Observe the mesh has a lot of black artifacts and invalidation

Tested with versions:
Unity 2018.1.0a1