speed improvements

• Back-ported from the new C/C++ v5 to Rust down-sampling of the spectrum when the number of cube channels exceeds the width of the browser window. Previously this downsizing step had been handled client-side in a web browser. The new version moves down-sampling server-side prior to sending the down-sampled spectrum over the network to the end users, which in many cases will result in a reduction in the bandwidth usage (smoother real-time spectrum updates over slow/unreliable network connections), plus a lesser CPU demand due to omitting the downsizing step client-side in JavaScript.
• Fixes a compilation warning when using Rust ≥ 1.44 (unused braces).