wibox: fix align

awful.wibox.align() in 3.4.x gets the available screen area from
capi.screen[screen].geometry.

this can easily result in overlapping wiboxes since the work area
provided does not account for existing wiboxes.

In my configuration I use a vertical wibox positioned at the
right and it half-overlaps my top wibox because it is streched and
hard-coded to align at the middle.

Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/wibox.lua.in

@@ -144,7 +144,7 @@ end
 function align(wibox, align, screen)
     local position = get_position(wibox)
     local screen = screen or wibox.screen or 1
-    local area = capi.screen[screen].geometry
+    local area = capi.screen[screen].workarea
 
     if position == "right" then
         if align == "right" then