plugins/glide: drop references to closed windows if they're not animated
Otherwise the window might still be referenced from the opening animation, which can lead to the effect wrongly keeping a reference to the window and staying active.
Easy way to reproduce this issue is to reduce animation speed to the minimum, open a window and then immediately close it again in the Overview.
Only CCBUG because I neither managed to trigger the issue with the scale effect so far, nor do I fully understand why the window gets stuck with the glide effect - in theory, the animation should complete after some time and the effect should clean things up even without this change.