broadway: Restore guint32 type for memory size

guint32 is used as part of the protocol in broadway backend.
Memory size declared with it was mistakenly replaced with size_t type
which does not guarantee being 32bit on all platforms, leading to a crash.

gdk/broadway/gdkbroadway-server.c

@@ -235,7 +235,7 @@ static void
 parse_all_input (GdkBroadwayServer *server)
 {
   guint8 *p, *end;
-  size_t size;
+  guint32 size;
   BroadwayReply *reply;
 
   p = server->recv_buffer;