hidpi: Nicer painting of the waved lines.

Change-Id: I8773b47967bc1aa8cf33b9a1edc4f826291d3554
Reviewed-on: https://gerrit.libreoffice.org/8518
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>