layer_surface_at: Fix the order of surface processing to match rendering
Commit 0064831b changed order of surface processing - however, by a mistake, the order in desktop's layer_surface_at didn't actually match the order of rendering nor the order described in the commit message.
The best way to test it is to pop out virtboard and lock the screen (the key is to have one surface with positive exclusive-zone and second, later one with exclusive-zone = -1 on the same layer). The keyboard is visible, but without this patch, input events go to the lockscreen.
See also: #67 (I swear I didn't put this bug in just to make the point! :P)