window-x11: Fix height computation of shaded windows

Since commit 6e06648f, we start out with the invisible frame parts
only, and then add the unconstrained rect's height (which consists of
the visible parts of both frame and client window) *unless* the window
is shaded. While we indeed don't want to add the client height in that
case, we need to explicitly include the visible frame parts now.

https://bugzilla.gnome.org/show_bug.cgi?id=746145