Fixes invisible mobs getting visible_message() blind message.

Fixes  #6715, hoping it doesn't add too much overhead and that it doesn't break #6207 too much (apparently you can see visible_message() from someone in closets...).

@phil235 
This would mean the src is sometimes not added to the list. You need to keep the `atom_viewers |= src` because the src must always.

You also need to add your line to  /atom/proc/visible_message().