Uses std::shared_ptr for sharing graphic data

Changed constructor to std::unique_ptr<sal_uInt8[]> to ensure
the delete[] operator is called when GfxLink internals takes
ownership of the data

Change-Id: I4edd4634df8d6ba4d94953260c1a7ac560ccf04a
Reviewed-on: https://gerrit.libreoffice.org/25402Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>