The only secure and robust way to check that a userdata is of the
expected type is to check its metatable. Userdata metatables are not
changeable by Lua code without the debug library, so if the metatable is
a certain table, only C code could have made it so. GLib type checking
functions are _not_ a robust or secure way to check that a block of
memory is a specific type of GObject, because Lua code could cause a
type confusion bug and potentially use it to forge pointers.
