Convert the pimpled copy-on-write Gradient class from vcl to use
::o3tl::cow_wrapper using the default reference counting policy.

Change-Id: Iadf4d2288669e58a4d5b41f436978c3ab34216f3
Reviewed-on: https://gerrit.libreoffice.org/18124Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>