56f6ac5f introduced a 20px shadow for the '.tiled decoration:backdrop'
selector. This selector is more specific than the '.ssd decoration'
selector and caused unfocused tiled SSD windows to have a shadow that
focused ones did not have. The mutter code however assumes that the size
of the decoration does not change between the states and this was
causing a crash when clicking below the titlebar in unfocused tiled SSD
windows.

Fixes #3716