Regression from commit e8d5b8be
(tdf#113714 vcl menu bar window: avoid flicker, 2019-05-20), the problem
was that a freshly created VirtualDevice doesn't have the default text
color, so we need to initialize that explicitly based on the render
context text color.

Also introduce a BufferDevice to do this initialization, instead of
fixing this in MenuBarWindow::Paint(), then copy&pasting the fix to
MenuFloatingWindow::Paint().

Change-Id: Ib171cd52e7cabe0bc3c639821f558d8303039fe6
Reviewed-on: https://gerrit.libreoffice.org/73269Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins