This was evidently a typo. Didn't get a crash but GCC 12 raised a
`-Warray-bounds` warning:
```
In file included from ./core/io/stream_peer.h:34,
from ./core/io/packet_peer.h:34,
from ./core/multiplayer/multiplayer_peer.h:34,
from modules/webrtc/webrtc_multiplayer_peer.h:34,
from modules/webrtc/webrtc_multiplayer_peer.cpp:31:
In member function 'T* Ref<T>::operator->() [with T = WebRTCMultiplayerPeer::ConnectedPeer]',
inlined from 'virtual Error WebRTCMultiplayerPeer::put_packet(const uint8_t*, int)' at modules/webrtc/webrtc_multiplayer_peer.cpp:376:4:
./core/object/ref_counted.h:101:24: error: array subscript 0 is outside array bounds of 'Ref<WebRTCMultiplayerPeer::ConnectedPeer> [0]' [-Werror=array-bounds]
101 | return reference;
| ^~~~~~~~~
```
Due to the async nature of WebRTC implementations, the multiplayer peer
could end up having queued packets from a given connection before it is
able to emit the "peer_added" signal.
This commit ensures that packets from peers which are not notified yet
are skipped by `get_packet` and `get_available_packet_count`.
Move multiplayer classes to "core/multiplayer" subdir.
Move the RPCConfig and enums (TransferMode, RPCMode) to a separate
file (multiplayer.h), and bind them to the global namespace.
Move the RPC handling code to its own class (RPCManager).
Renames "get_rpc_sender_id" to "get_remote_sender_id".
* Added a new macro SNAME() that constructs and caches a local stringname.
* Subsequent usages use the cached version.
* Since these use a global static variable, a second refcounter of static usages need to be kept for cleanup time.
* Replaced all theme usages by this new macro.
* Replace all signal emission usages by this new macro.
* Replace all call_deferred usages by this new macro.
This is part of ongoing work to optimize GUI and the editor.