add opcode field to packets

X11 requests must contain an opcode field
to be sent so the X11 server can identify
the request

lib/X11/packet.rb

@@ -25,7 +25,7 @@ module X11
             end
           end
 
-          packet.join
+          ((@opcode ? [X11::Type::Int8.pack(@opcode)] : []) + packet).join
         end
 
         def read(socket)
@@ -73,6 +73,10 @@ module X11
           @structs << s
         end
 
+        def opcode(value)
+          @opcode = value
+        end
+
         private
 
         def lengths_for(args)