gtk4/theme: Use 0.5 opacity for disabled pictures

If a GtkImage is using an icon we use a gtk-icon-filter to se the icon
opacity when in insensitive state, however when using other kinds of
pictures we do not apply the same style leading to an inconsistent
result.

Sync with upstream change: https://github.com/GNOME/gtk/commit/a2076dd537544813b9936bd9e79654f3401c43b2


(cherry picked from commit e11570e5)