The high-level problem was that a watermark shape in the background was
rendered with lighter and darker gray as the user typed. The reason for
this was that depending on what larger combined tile was rendered we did
or did not repaint the layout frame.

Handle the situation similar to when we have no vcl::Window at all,
which ensures that we always paint only once. The rgb value matches the
desktop rendering result this way.

(Just assert that we render the gray light enough, the actual color
channel value may be 190 or 191.)

Change-Id: Ie8746ab70f49f7f1080632c39e3a826c4ce509df
Reviewed-on: https://gerrit.libreoffice.org/72276Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins