Guard against calling set_size while applying a configure event more explicitly

Depending on code path taken, geometry.size() == m_pendingSize can produce incorrect results.

If a configure event is applied, it's fine.

If the window is resized by user, m_pendingSize will have outdated value, and setWindowGeometry() can ignore future size updates that are valid.

In hindsight, we need special hooks in the QWaylandWindow to request and apply new geometry. Rather than have one function that deals with all cases.