record all Windows, add Window.event_loop

runtime/lib/window.rb

@@ -1,7 +1,26 @@
+require "set"
+
 class Window
+  @windows = Set.new
+
+  class << self
+    def register(window)
+      @windows << window
+    end
+
+    def remove(window)
+      @windows.delete(window)
+    end
+  end
+
   def initialize
     @programs = {}
     load_shaders
+    Window.register(self)
+  end
+
+  def close
+    Window.remove(self)
   end
 
   private