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/node_globals_test.cpp

@@ -17,7 +17,7 @@ TEST(NodeGlobals, ConstructedWithParams)
 	olive::VideoParams video_params(1920, 1080, olive::PixelFormat::F32, 4);
 	olive::AudioParams audio_params;
 	audio_params.set_sample_rate(48000);
-	audio_params.set_channel_layout(AV_CH_LAYOUT_STEREO);
+	audio_params.set_channel_layout(olive::core::kChannelLayoutStereo);
 
 	olive::TimeRange time(olive::core::rational(1, 24),
 						  olive::core::rational(2, 24));