SteamNetworkingMessage_t isn't an unmanaged type, blocking optimized API calls

[Idles]

The C# definition for SteamNetworkingMessage_t is (accidentally?) a managed type, which necessitates an unneeded GC allocation and some copies when using it to call SteamNetworkingSockets.SendMessages. See the sample code provided in #598.

The root cause making it a managed type is its SteamNetworkingIdentity field, which is itself a managed type because of its field m_reserved. Until struct inline arrays arrive and can be broadly used (incl. Unity), replacing m_reserved with 32 individual uint fields transitively makes SteamNetworkingMessage_t an unmanaged type.

This one (tedious) code change in SteamNetworkingIdentity allows an optimized call to SteamNetworkingSockets.SendMessages shown in the (unsafe) sample code below. I've tested it successfully at runtime using my other local modifications to Steamworks.NET discussed in #598.

var ptr = SteamNetworkingUtils.AllocateMessage(messageBytes.Length);
var msg = (SteamNetworkingMessage_t*)ptr.ToPointer();

Marshal.Copy(messageBytes, 0, msg->m_pData, messageBytes.Length);
msg->m_nFlags = flags;
msg->m_idxLane = lane;
msg->m_conn = conn; 

var msgPtrsToSend = new IntPtr[] { ptr };
SteamNetworkingSockets.SendMessages(1, msgPtrsToSend, null);

[rlabrecque]

That makes complete sense, I wonder if there's any other shenanigans we can pull instead 🤔