Windows (by default) and gtk3 paints its own background, but e.g. the
Linux gen backend does not; so make sure that we not only copy from the
buffer, but also initialize it.

This restores the gradient background of the main menu with the Linux
gen backend.

(cherry picked from commit a2da909a)

Change-Id: I5ce8cc734f64bc1d57d343caf22071e6aa63a69f
Reviewed-on: https://gerrit.libreoffice.org/72685Tested-by: Xisco Faulí <xiscofauli@libreoffice.org>
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>