Fixes for window fade-outs

Fixes for window fade-outs

When a window resizes while it is unmapped, don't destroy it's pixmap, as that kills and fade going on.
If a window is already unmapped (and therefore already fading out, don't override the fadeout with a new one when the window is destroyed.
When a window maps, if it already has a pixmap, this is stale data, so free it so that a new pixmap can be retrieved from the newly mapped window.