awful.tooltip: skip set_geometry if not visible

`set_geometry` will be called in `show`, and is not necessary when the
tooltip is not visible.

lib/awful/tooltip.lua

@@ -130,7 +130,9 @@ end
 --   `wibox.widget.textbox.set_text`.
 tooltip.set_text = function(self, text)
     self.textbox:set_text(text)
+    if self.visible then
         set_geometry(self)
+    end
 end
 
 --- Change displayed text.
@@ -140,7 +142,9 @@ end
 --   `wibox.widget.textbox.set_markup`.
 tooltip.set_markup = function(self, text)
     self.textbox:set_markup(text)
+    if self.visible then
         set_geometry(self)
+    end
 end
 
 --- Change the tooltip's update interval.