libweston: send more wl_surface.enter/leave events

A client may have bound the same wl_output multiple times, for who knows
what reason. As the server cannot know which wl_output resource to use
for which wl_surface, send enter/leave events for all of them.

This is a protocol correctness fix.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>