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/codec/encoder.cpp

@@ -300,7 +300,7 @@ void EncodingParams::Save(QXmlStreamWriter *writer) const
 
 		writer->writeTextElement(
 			QStringLiteral("channellayout"),
-			QString::number(audio_params().channel_layout().u.mask));
+			QString::number(audio_params().channel_layout()));
 		writer->writeTextElement(
 			QStringLiteral("format"),
 			QString::fromStdString(audio_params_.format().to_string()));
@@ -542,12 +542,8 @@ bool EncodingParams::LoadV1(QXmlStreamReader *reader)
 					audio_params_.set_sample_rate(
 						reader->readElementText().toInt());
 				} else if (reader->name() == QStringLiteral("channellayout")) {
-					AVChannelLayout av_channel_layout;
-					av_channel_layout_from_mask(
-						&av_channel_layout,
+					audio_params_.set_channel_layout(
 						reader->readElementText().toLongLong());
-					audio_params_.set_channel_layout(av_channel_layout);
-					av_channel_layout_uninit(&av_channel_layout);
 				} else if (reader->name() == QStringLiteral("format")) {
 					audio_params_.set_format(SampleFormat::from_string(
 						reader->readElementText().toStdString()));