* Add support for c++20 where available.
* residfp: retuned cw parameters after measuring waves RMS.
* residfp: code refactoring and minor performance optimizations.
* resid/residfp: scale down filter inputs in the mixer for 6581.
* Centralized handling of muted voices and allow muting samples.
* new API to toggle the filter and deprecated sidplayfp::filter.
OBS-URL: https://build.opensuse.org/package/show/multimedia:libs/libsidplayfp?expand=0&rev=28
