Code review comment for lp:~townsend/unity/fix-lp1064155

Like the compiz merge proposal, I feel like this is probably in the wrong place. The correct approach would be to not add the minimized window to fullscreen_windows_ in the first place in UnityWindow::glPaint rather than checking for a minimized fullscreen window in fullscren_windows_ later and preventing the operation of the occlusion code after that.