src/ssd: use view->ssd_titlebar_hidden for ssd_thickness calculations

src/ssd: use view->ssd_titlebar_hidden for ssd_thickness calculations

Before this patch we were using the internal .enabled flag of the titlebar
tree node. This failed due to ssd_thickness() not having view->ssd assigned
when initially called. Instead of assigning view->ssd within ssd_create()
we just always use the view boolean flag directly. This fixes an issue
where a border-only view that has been snapped to an edge or region would
have a gap in the size of the titlebar on top after a Reconfigure.

Fixes #1083