FFmpegAudioDecoder only used swresample to turn the decoders' output into
interleaved stereo s16, at the same sample rate. A small conversion does that
now, with swresample's rounding: floats go through lrintf and are clamped, and
mono goes to both channels at -3 dB, the coefficient swresample's mixing matrix
uses. Its output is sample-identical to swresample's for the MP3, AAC and
ATRAC3+ decoders, mono and stereo.
With "Use FFMPEG for all compressed audio", ATRAC3 got closed on its first
frame: its decoder sets no channel layout, and swresample refused to set up
without one. The conversion goes by the channel count, so that no longer fails.
The system-FFmpeg build no longer looks for libswresample.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
