Shadows on popups before initial rendering

[David Edmundson]

I should probably have explained what my patch does in my email.

I add an XAtom to Dialog. This Atom is set to true between
construction and the first texture being rendered which we can tell
from QQuickWindow then it is deleted.

Then I patched the slidingPopup effect to look for this atom and
suppress painting the window (and therefore shadows) whilst this atom
is set.

I did it this way, so that any other system using sliding popups but
not setting my _KDE_PENDING atom will still work as before.

It seems to work.