effects/overview: fix behavior when deskotp grid doesn't layout
When the option "organize windows in desktop grid" is disabled, the windows on other desktops were invisible (and opacity===0)
Also, the offsetX property wasn't set quite right, because
initialHidden now does not depend on isOnOtherDesktop anymore so the windows are visible, and offsetX depends on mainBackground.deltaColumn when gridVal is > 0, is not inversely proportional to gridval anymore, but on/off