tests: migrate gtest suite to the ffmpeg_bridge API and add bridge coverage

- Replace direct FFmpeg usage in the test suite (channel layout masks,
  pixel/sample format constants, AVFrame field access, sws_scale) with
  the bridge equivalents and the olive::AVFrame adapter
- Drop CoreRational.ToAVRational (API removed with core's FFmpeg
  dependency) and the GetSwsColorspaceFromAVColorSpace tests (helper
  moved inside the bridge)
- New ffmpeg_bridge_test.cpp exercises the C API directly: constants vs
  core, error strings, pixel format utilities, frames/packets, scaler,
  resampler, audio graph tempo processing, probe/decoder round-trip on
  tests/demo.mp4 (including hw-frame transfer), SRT subtitle reading,
  and encoder end-to-end tests (PNG video and PCM audio probed back)

tests/gtest/audio_waveform_sync_test.cpp

@@ -5,16 +5,12 @@
 #include "olive/core/render/samplebuffer.h"
 #include "olive/core/render/sampleformat.h"
 
-extern "C" {
-#include <libavutil/channel_layout.h>
-}
-
 namespace
 {
 
 olive::core::AudioParams MakeMonoParams()
 {
-	return olive::core::AudioParams(48000, AV_CH_LAYOUT_MONO,
+	return olive::core::AudioParams(48000, olive::core::kChannelLayoutMono,
 									olive::core::SampleFormat::F32P);
 }