Initialize workspace rect to the output's upon creation

Initialize workspace rect to the output's upon creation

The comment immediately following implied that this was the intended
behaviour. Not doing so means that compound commands that both move a
window to a new workspace as well as do something that depends on the
workspace's geometry (e.g. 'move position center' or 'floating enable'
on a tiled window) would use the workspace's calloc'd 0x0+0x0 geometry.