Fix FFMpeg API compatibility for modern FFMpeg 6.x

[Copilot]

This PR resolves issue #78 by updating FFMpegWrapper.cpp to use the modern FFMpeg API, fixing compilation errors with FFMpeg 6.x.

Issues Fixed

The original code used deprecated FFMpeg APIs that were removed in recent versions:

1. av_register_all() removed: This function was deprecated in FFMpeg 4.0 and removed in 5.0+
2. Type mismatch: avcodec_find_decoder() now returns const AVCodec * instead of AVCodec *
3. Channel API changes: channels field replaced with ch_layout.nb_channels in both AVCodecParameters and AVFrame
4. Channel layout functions: av_get_default_channel_layout() deprecated in favor of av_channel_layout_default()

Changes Made

• Removed deprecated registration: Replaced av_register_all() call with explanatory comment (auto-registration since FFMpeg 4.0)
• Fixed decoder type: Changed AVCodec *decoder to const AVCodec *decoder
• Updated channel access: Replaced all instances of ->channels with ->ch_layout.nb_channels
• Modernized channel layout setup:
  • Replaced channel_layout field checks with ch_layout equivalents
  • Updated from av_get_default_channel_layout() to av_channel_layout_default()
  • Changed SWR context setup from av_opt_set_int() with *_channel_layout to av_opt_set_chlayout() with *_chlayout
• Added required header: Included <libavutil/channel_layout.h> for new channel layout API

Testing

• Successfully compiles with FFMpeg 6.1.1 (libavcodec 60.31.102)
• All changes maintain existing functionality while using the modern API
• Built and tested anmp-dump binary executes correctly

Backward Compatibility

These changes target FFMpeg 4.0+ API. The new channel layout API was introduced in FFMpeg 5.1, but the other changes (removing av_register_all(), const correctness) are compatible with FFMpeg 4.0+.

Closes #78

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