x11: Fix shading with non-zero border

There was a geometry change that fixed mixing the next and current
geometries. While it did fix issues on wayland, it broke window shading
on x11 because of an obscure resize() call.

That obscure resize() had a side-effect that ensures m_clientGeometry
has the right value so the next time the window is unshaded,
implicitSize() will return a good value.

In order to make window size computation more robust, this change makes
X11Window compute the natural frame size based on cached size in
m_client, which shouldn't change when the window is shaded.

However, given how buggy window shading is and how difficult it is to
make it work right, I think that it's better to deprecate window shading
and remove it in some future release.

BUG: 450582
(cherry picked from commit 4790916f)