Add userdata to lua_Callbacks

[LoganDark]

My use case for this is storing data that functions in lua_Callbacks can access, but also anything else that needs VM-wide state rather than thread-local.

[zeux]

Does L->global->mainthread->userdata work for this use case? If not then I'd probably go with userdata in lua_Callbacks as that way we won't need to invent another way to access this through the official C API.

[LoganDark]

Does L->global->mainthread->userdata work for this use case?

Not really. This is for userdata global to the whole VM.

If not then I'd probably go with userdata in lua_Callbacks as that way we won't need to invent another way to access this through the official C API.

Are applications not supposed to access global_State directly?

Whoops. I forgot that global_State is apparently a private implementation detail. I generate bindings for all the headers in VM, but you're not technically supposed to do that. ;)