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/core_samplebuffer_test.cpp

@@ -6,7 +6,7 @@ using namespace olive::core;
 
 static AudioParams MakeParams(int channels = 2, int sample_rate = 48000)
 {
-	AudioParams params(sample_rate, AV_CH_LAYOUT_STEREO, SampleFormat::F32P);
+	AudioParams params(sample_rate, kChannelLayoutStereo, SampleFormat::F32P);
 	return params;
 }