Regression from commit 458a827e (further
refactor Menu to use RenderContext, 2015-05-15), if we do full paint
instead of incremental paint, then need to ensure that an intermediate
state is not painted.

Paint happens at idle time by default on Windows (OpenGL) and also on
Linux (gtk3), but some other backends like Windows GDI had flicker, this
fixes the problem.

Change-Id: I530166cea93513aec4648dd0a385359f4b998b6f
Reviewed-on: https://gerrit.libreoffice.org/72610Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins