wibox.drawable: Do not relayout while invisible

Similar to the previous commit, this makes the drawable not apply a
pending relayout while it is not visible. When it becomes visible again,
the relayout is done.

The hope here is that less work is done while a drawable is not visible,
saving CPU time.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/drawable.lua

@@ -429,7 +429,11 @@ function drawable.new(d, widget_context_skeleton, drawable_name)
             return
         end
         ret._need_relayout = true
-        ret:draw()
+        -- When not visible, we will be redrawn when we become visible. In the
+        -- mean-time, the layout does not matter much.
+        if ret._visible then
+            ret:draw()
+        end
     end
 
     -- Add __tostring method to metatable.