Exported OBJ materials are transparent instead of opaque

[brettle]

For opaque materials, Tr should be 0.0 not 1.0 (see here). The problem appears to be here:

vcglib/wrap/io_trimesh/io_material.h

Line 103 in e4950d1

mtl.Tr = (float)(f.C()[3])/255.0f;//alpha

[alemuntoni]

fixed! Thanks!