WindowThumbnail: postpone texture deletion to avoid crash
The culprit was the texture was deleted as soon as the item was deleted while it may be used a bit longer by the render thread.
See also: https://github.com/sailfishos/sailfish-office/pull/85
BUG: 464186 BUG: 446874 FIXED-IN: 5.104
Edited by Fushan Wen