compositor: fix redraw when switching with minimized surfaces

With DRM, minimized surfaces would occasionate flicker when
shown and re-hidden during switch with Mod-Tab.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
shell: fix taskbar updates with multiple outputs

Taskbars mirrored on additional outputs were not correctly
updated along with surfaces.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>