cairo canvas: Zero font's width means "the same as height".

Change-Id: I1d24b2039fb3c615e672189d12c77e960cc858ee
Reviewed-on: https://gerrit.libreoffice.org/7763
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>