Merge lp://qastaging/~3v1n0/unity/panel-shadow-draw-fixes into lp://qastaging/unity

UnityWindow: properly draw the panel shadow when a Nux window is focused or during WS switch

If a nux window is the active one, compiz won't draw it, so the code that is doing the
panelShadow drawing is excluded. We need to make sure that this will be executed
also in this case.

Optimizations: unity at every window redraw was executing a loop to check if a window was or not a nux one. Now we set this value on UnityWindow initialization, making the check a lot less expensive.
Removed some uneeded initializations on paintPanelShadow, and using uscreen's GetMonitorAtPosition instead of an extra loop.

Fixes: nux windows are currently excluded from the compiz draw process, this lead to not executing the paintPanelShadow code into UnityWindow::glDraw, so we need to call this in glPaint to get the proper panel shadow to be drawn when the launcher, the dash/hud or shutdown dialogues are fucused.
Also, making the panel shadow to redraw properly on transformed output.