wayland: Fix getting the last configure event
If there's only one configure event that changes the position of the window and it gets acknowledged but no buffer is attached yet, and a new configure is sent, then the ConfigurePosition flag won't be inherited by the new configure event and the window will be misplaced.
In order to fix that, this change makes XdgSurfaceClient pop the last acknowledged configure event from the m_configureEvents list only when it's about to be applied for sure.
Edited by Vlad Zahorodnii