Fix horizontal-line SvgItem
Amends 1accb119
SvgItem's preferredWidth was bound to a scrollView.viewport.width which
should've been replaced with scrollView.availableWidth when porting
ScrollArea to ScrollView, like in other places. But it doesn't matter,
as Layout.fillWidth: true
should've been used instead anyway.
I didn't fix much with this commit, but at least it didn't make things worse :)