convert number of "lines" scrolled to double and allow
fractional parts of lines/columns

Change-Id: Ib99c815cfc8823e22fc1d76e201903c34ed0f61b
Reviewed-on: https://gerrit.libreoffice.org/37669Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>