shims: Fix the screen workarea value.

It didn't work with multiple screens.

tests/examples/shims/screen.lua

@@ -1,23 +1,37 @@
 local gears_obj = require("gears.object")
 local gears_tab = require("gears.table")
+local grect = require("gears.geometry").rectangle
 
 local screen, meta = awesome._shim_fake_class()
 screen._count, screen._deleted = 0, {}
 
+local function get_drawin_screen(s, d)
+    return grect.area_intersect_area (s.geometry, {
+        x      = d:geometry().x,
+        y      = d:geometry().y,
+        width  = 1,
+        height = 1
+    })
+end
+
 local function compute_workarea(s)
     local struts = {top=0,bottom=0,left=0,right=0}
 
     for _, c in ipairs(drawin.get()) do
+        if get_drawin_screen(s, c) then
             for k,v in pairs(struts) do
                 struts[k] = v + (c:struts()[k] or 0)
             end
         end
+    end
 
     for _, c in ipairs(client.get()) do
+        if c.screen == s then
             for k,v in pairs(struts) do
                 struts[k] = v + (c:struts()[k] or 0)
             end
         end
+    end
 
     return {
         x      = s.geometry.x      + struts.left,