Add in a little rectangle placeholder.

control.cpp

@@ -31,9 +31,11 @@ void ControlUI::init() {
   $gui.set<guecs::Text>(docs_id, {L"A: win left\nD: win right\nQ: quit"});
 
   auto current = $gui.entity("current");
+  $gui.set<guecs::Rectangle>(current, {});
   $gui.set<guecs::Text>(current, {L"Current Slide"});
 
   auto next = $gui.entity("next");
+  $gui.set<guecs::Rectangle>(next, {});
   $gui.set<guecs::Text>(next, {L"Next Slide"});
 
   $gui.init();