Server: avoid per-frame deep copy of audio buffers in CreateLevelsForAllConChannels

[mcfnord]

What

CServer::CreateLevelsForAllConChannels() declares its vecvecsData parameter as

const CVector<CVector<int16_t>> vecvecsData   // by value

This PR changes it to const CVector<CVector<int16_t>>& (by const reference), matching the neighbouring vecNumAudioChannels parameter.

Why

The function is called from CServer::OnTimer() — the realtime audio timer thread — on every frame in which at least one client is connected:

const bool bSendChannelLevels = CreateLevelsForAllConChannels ( iNumClients, vecNumAudioChannels, vecvecsData, vecChannelLevels );

Because CVector derives from std::vector, passing by value deep-copies the entire pre-allocated member vector every frame. vecvecsData is initialized in the CServer constructor to iMaxNumChannels (the -u setting) worst-case stereo buffers and is never shrunk, so the copy always covers the full configured channel limit — even a nearly empty server pays it in full. This both allocates on the audio thread (which the codebase otherwise takes care to avoid — see the "no memory must be allocated" comments in the CServer constructor) and burns memory bandwidth.

A small allocation-counting reproduction of the exact copy (150 channels = -u 150, 2×128 int16 per buffer, 375 frames/s) measured ~56,600 heap allocations/s and ~30 MB/s copied, purely from the by-value parameter; at the default -u 10 it is still ~4,100 allocations/s and ~2 MB/s. The copy is not elided at -O2.

Change

One parameter, declaration + definition — no behavioural change. The function only reads vecvecsData.

Testing

• Headless build (CONFIG+=headless, Qt 5.15) is clean.
• Server starts, runs, and shuts down cleanly on SIGTERM (OnHandledSignal: 15).
• clang-format-14 applied to the touched files.

See #3804 for the analysis.

[ann0see]

Seems sensible. We'd need to check if there's really no reason for a copy though.

[mcfnord]

Not sure how to parse the scope of const here, but it might be a clue that perhaps there's no reason to copy the bits.