[fuseCut] Exported tetrahedralization debug meshes have no colors

[almarouk]

The problem is caused by the save function of class Mesh that doesn't take into consideration the colors when creating the Assimp Scene.

https://github.com/alicevision/AliceVision/blob/3472774336c589085b028a62b3dbd7e248f6f088/src/aliceVision/mesh/Mesh.cpp#L87