Migrate (some) of the UDIM offset calculation from inside one
of the packing engines (where it's consumed) to the packing
operator (where it's produced).
This change (and others) will help simplify the future migration
of the packing engine inside editors/uvedit/uvedit_islands.cc
to the Geometry module, so it can eventually replace the other
packing engine in geometry/intern/uv_parametrizer.cc
0e90896cba