skins2: fix art display misfunctioning in latest git vlc

Art display used to rely on an event issued by vlc core which has been renamed into "activity" lately. The issue was that it seems no longer fired when a new
 input is launched.
skins2: fix art file failing to display when changing skin theme

When changing a skin theme on the fly, an input may already be running
 and the art file is expected to display in the new theme.

This patch takes into account a possible art file already kwown
at initialisation of a new image control with art enabled.
modules: do not use non-portable union to store item flags in cache

 Pointed-out-by: Mario Speiß <1034-135@online.de>