qt: consider csd shadows when requesting blur behind effect

KWindowSystem's documentation states the following:

Note that you will usually want to set the region to the shape of the window, excluding any shadow or halo.

I interpret this as, the requested blur region should not encapsulate the area where the window shadows are drawn by the client (application) because client shadows are part of the window.

@chub Currently on Plasma/KWin/KWindowSystem 6.2, the blur seems to be applied also to the client shadow area. It is not clear to me why this did not happen before.