client: Invoke new_id closure arguments as pointers instead of integers

This commit adds a flags parameter to wl_closure_invoke(). The so far
added flags are ment to specify if the invokation is client side or
server side. When on the server side, closure arguments of type 'new_id'
should be invoked as a integer id while on the client side they should
be invoked as a pointer to a proxy object.

This fixes a bug happening when the address of a client side 'new_id'
proxy object did not fit in a 32 bit integer.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>