C++26 adds std::saturate_cast, so follow the established pattern of
other similar "backported" APIs. The old name is left around while
we port other submodules.
While at it, move qt_saturate's tests to the qnumeric test.
Change-Id: I653a2e3d936081378298a9c8e51e7c1a2d438d83
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 7447ad503330ed176cf369792ffb33b7e00a58d3)
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
27dc042712