wibox.drawable: Force full repaint when the context changes

The previous commit made the hierarchy do a re-layout when the context changes.
However, widgets could change their appearance depending on the context without
changing their layout. Thus, the previous commit is not enough.

This commit also makes the drawable redraw everything when the context changes.

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

lib/wibox/drawable.lua

@@ -51,6 +51,9 @@ local function get_widget_context(self)
             context[k] = v
         end
         self._widget_context = context
+
+        -- Give widgets a chance to react to the new context
+        self._need_complete_repaint = true
     end
     return context
 end