Fix T81756: relinking multiple links to group input removes links

The links where added to the socket one after the other. However,
the virtual socket had a link limit of 1, so whenever a new link was
added, the previously added one was removed.

There is not really a reason for why the link limit should be 1 instead
of something higher. I'm setting it to the max value: `0xFFF`.

I'm also setting the `input_link_limit` to that value. Blender does not
need this currently, but addons might have input sockets that allow
more than one incident link.

source/blender/nodes/intern/node_socket.cc

@@ -543,8 +543,8 @@ static bNodeSocketType *make_socket_type_virtual(void)
   ED_init_node_socket_type_virtual(stype);
 
   stype->use_link_limits_of_type = true;
-  stype->input_link_limit = 1;
-  stype->output_link_limit = 1;
+  stype->input_link_limit = 0xFFF;
+  stype->output_link_limit = 0xFFF;
 
   return stype;
 }