app: route all FFmpeg access through the ffmpeg_bridge C API

Replace every direct FFmpeg call in the editor and the render worker
with the pure C ffmpeg_bridge API, wrapped in thin C++ adapters that
preserve the original interfaces:

- avframeptr.h: olive::AVFrame adapter around FBFrame handles
- ffmpegutils: format conversion helpers on FB_* constants; the int
  overload is renamed GetCompatibleBridgePixelFormat to avoid a silent
  overload-resolution trap with the PixelFormat enum
- ffmpegdecoder/ffmpegencoder: rewritten as handle-based adapters over
  FBDecoder/FBProbe/FBEncoder/FBScaler/FBResampler
- audioprocessor: FBAudioGraph push/pull adapter
- pluginrenderer/OliveClip: sws/pixdesc usage converted to FBScaler and
  fb_pix_fmt_* queries
- AudioParams/channel layouts are plain uint64_t masks everywhere

Build integration: the root project no longer links FFMPEG directly;
only ffmpeg_bridge does. Binaries resolve the bridge library at runtime
via @loader_path inside the macOS app bundle (copied there post-build)
and via $ORIGIN/../ffmpeg_bridge/bin on Linux; on Windows the DLL is
installed next to the executables, so packages on all three platforms
ship the bridge library.

ffmpeg_bridge gains the extra API the adapters need:
fb_frame_make_writable, fb_decoder_get_format_duration,
fb_resampler_convert_frame, FB_PIX_FMT_YUV440P, SRT validation in
fb_probe_read_subtitle_stream, packed/planar fixes in
fb_encoder_write_audio, and a component-size fix in
fb_pix_fmt_component_size.

app/node/output/viewer/viewer.cpp

@@ -236,12 +236,10 @@ void ViewerOutput::set_default_parameters()
 		OLIVE_CONFIG("DefaultSequenceInterlacing")
 			.value<VideoParams::Interlacing>(),
 		1));
-	AVChannelLayout layout;
-	av_channel_layout_from_mask(
-		&layout, OLIVE_CONFIG("DefaultSequenceAudioLayout").toULongLong());
 	SetAudioParams(
 		AudioParams(OLIVE_CONFIG("DefaultSequenceAudioFrequency").toInt(),
-					layout, kDefaultSampleFormat));
+					OLIVE_CONFIG("DefaultSequenceAudioLayout").toULongLong(),
+					kDefaultSampleFormat));
 }
 
 void ViewerOutput::InvalidateCache(const TimeRange &range, const QString &from,

app/node/project/serializer/typeserializer.cpp

@@ -59,7 +59,7 @@ void TypeSerializer::SaveAudioParams(QXmlStreamWriter *writer,
 	writer->writeTextElement(QStringLiteral("samplerate"),
 							 QString::number(a.sample_rate()));
 	writer->writeTextElement(QStringLiteral("channellayout"),
-							 QString::number(a.channel_layout().u.mask));
+							 QString::number(a.channel_layout()));
 	writer->writeTextElement(QStringLiteral("format"),
 							 QString::fromStdString(a.format().to_string()));
 	writer->writeTextElement(QStringLiteral("enabled"),