Close files at rec-stop (re-open as needed)

This is in preparation to allow recording .flac files which are either
read or write not cannot be in read/write mode

libs/ardour/file_source.cc

@@ -519,6 +519,7 @@ FileSource::mark_immutable ()
 	/* destructive sources stay writable, and their other flags don't change.  */
 	if (!(_flags & Destructive)) {
 		_flags = Flag (_flags & ~(Writable|Removable|RemovableIfEmpty|RemoveAtDestroy|CanRename));
+		close();
 	}
 }