Match Core Audio channel layouts for FLAC

[sbooth]

The channel layouts for FLAC 4.0, 5.0, and 5.1 are possibly incorrect.

4 channels

ffmpeg uses the quad layout.

CoreAudio uses kAudioChannelLayoutTag_WAVE_4_0_B (L R Rls Rrs). This is the current behavior.

It might be more correct to use kAudioChannelLayoutTag_Quadraphonic (L R Ls Rs -- 90 degree speaker separation).

5 channels (5.0)

CoreAudio uses kAudioChannelLayoutTag_WAVE_5_0_A (L R C Ls Rs).

This differs from the current behavior which is kAudioChannelLayoutTag_WAVE_5_0_B (L R C Rls Rrs).

6 channels (5.1)

CoreAudio uses kAudioChannelLayoutTag_WAVE_5_1_A (L R C LFE Ls Rs).

This differs from the current behavior which is kAudioChannelLayoutTag_WAVE_5_1_B (L R C LFE Rls Rrs).

The Core Audio channel definitions are:

// Ls - left surround
// Rs - right surround
// Rls - rear left surround
// Rrs - rear right surround

[Copilot]

Pull request overview

This PR updates the channel layout mappings for FLAC files to better align with Core Audio's standard WAVE channel layout conventions. Specifically, it changes the decoder to use the "A" variants of WAVE layouts (which use standard surround positions Ls/Rs) instead of the "B" variants (which use rear surround positions Rls/Rrs), and switches from MPEG_3_0_A to WAVE_3_0 for 3-channel audio.

Changes:

• Changed 3-channel layout from MPEG_3_0_A to WAVE_3_0 for better consistency with WAVE conventions
• Changed 5-channel layout from WAVE_5_0_B (L R C Rls Rrs) to WAVE_5_0_A (L R C Ls Rs)
• Changed 6-channel layout from WAVE_5_1_B (L R C LFE Rls Rrs) to WAVE_5_1_A (L R C LFE Ls Rs)

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