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_rational_test.cpp

@@ -82,13 +82,6 @@ TEST(CoreRational, ToString)
 	EXPECT_EQ(rational(1, 2).toString(), "1/2");
 }
 
-TEST(CoreRational, ToAVRational)
-{
-	AVRational av = rational(3, 4).toAVRational();
-	EXPECT_EQ(av.num, 3);
-	EXPECT_EQ(av.den, 4);
-}
-
 TEST(CoreRational, Arithmetic)
 {
 	rational a(1, 2);