libweston: Split notify_pointer focus into notify/clear

This lets us say what we really mean instead of passing a NULL output
and garbage co-ordinates.

This will help later when manufacturing garbage coordinates becomes much
harder to do.

The clear_pointer_focus() path continues to do nothing, and is just a FIXME
macro, as it has been for a very long time...

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>