xcb: Fix visuals iterator methods
xcb: Fix visuals iterator methods

https://bugzilla.gnome.org/show_bug.cgi?id=730090

GNOME/vala,rvu95/vala,GNOME/vala,frida/vala,spidermonkdust/vala,frida/vala,spidermonkdust/vala,frida/vala,rvu95/vala,flobrosch/vala-fixes,flobrosch/vala-fixes,spidermonkdust/vala,GNOME/vala,rvu95/vala,GNOME/vala,frida/vala,frida/vala,flobrosch/vala-fixes