compositor: Always set output_mask in weston_surface_update_output_mask().

compositor: Always set output_mask in weston_surface_update_output_mask().

Commit 982387011 causes a bug where starting weston results in a black screen
(if no clients are immediately started). The problem is that the offending
commit causes the compositor to not damage if a surface has an empty output
mask, which is the case for the fade surface, which is created by the
compositor.  This patch updates the surface output_mask unconditionally,
and only skips sending out the events if there no client.