onPlaybackSpeedChanged

Invoked whenever the playback rate changes, including the rate the engine restores by itself when net.bunny.bunnystreamplayer.config.PlaybackSpeedConfig.rememberLastSpeed is on.

Hosts that draw their own speed selector need this: the remembered rate is applied to the next video without anyone touching the UI, and a selector that only tracks its own taps ends up showing 1× over a video playing at 0.25×.