wayland: Fix focus follows mouse policy
We want to update the input focus only if the pointer is moved. Due to that, AbstractClient::enterEvent() checks the last seen pointer position to decide whether the window needs to be focused. The issue is that when the pointer moves from a decoration to a surface, the cached pointer position will be updated to the current pointer position, and thus the check in AbstractClient::enterEvent() will fail. We need to update the cached pointer position only if there is a focused decoration.