ext-session-lock: Do not use commit listener to arrange

Arranging lock surfaces rely on the sway_output width and height being
updated, but these are only updated after the commit has been completed
and all commit listeners have executed. This means that the lock
surfaces will not be appropriately scaled to match a change in output
dimensions, and may reveal what is under the lock background.

Replace the implicit arrange through the output commit listener with an
explicit arrange after the output configuration is finalized.

This might have regressed by other transition away from output commit
listeners for other arrange tasks, but even then it would have
erroneously relied on signalling order.

include/sway/lock.h

@@ -0,0 +1,6 @@
+#ifndef _SWAY_LOCK_H
+#define _SWAY_LOCK_H
+
+void arrange_locks(void);
+
+#endif