Discard window thumbnail pixmap after texture got destroyed by SceneGraph

If the window holding the WindowThumbnail item goes away the texture hold
by the node is destroyed but our bound window pixmap is not yet freed.
This results in incorrect state the next time the WindowThumbnail is
shown.

To get back into a clean state discardPixmap() is called if there is no
texture but a bound low level pixmap.


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