fdo#80014: vcl: fix drawing of linear gradients

There is an off-by-one error in OutputDevice::ImplDrawLinearGradient()
that causes the bottom line to remain black.

(regression from 04d937c1ec36c2d9fa8c90604c81a37d30e97da6)

Change-Id: I33948ec8065e446a9ec7efeab5e506c93fa2e9e8
(cherry picked from commit d51f53fedb8cd68b1b3122c7199c4ea2d3a6980f)
Reviewed-on: https://gerrit.libreoffice.org/10024
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>