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.