Consistently use ptsToSeconds

[Dan-Flores]

As discussed in #764, this PR updates various methods in SingleStreamDecoder.cpp to use ptsToSeconds when performing calculations with the time_base.

Additionally, ptsToSeconds is updated to use av_q2d to ensure the AVRationals are correctly handled.

Tests are updated as well to reflect the added precision:

• In VideoDecoderTest.cpp, comparisons of ptsSeconds are updated from EXPECT_EQ to EXPECT_DOUBLE_EQ.
• In test_ops.py, pytest.approx is used to compare pts_seconds.

[NicolasHug]

Thanks @Dan-Flores

[NicolasHug]

I'm surprised we need to use approx so just to be on the safe side I removed it and we're getting:

FAILED test/test_ops.py::TestAudioDecoderOps::test_next[asset0] - assert 0.5760000000000001 == 0.576
FAILED test/test_ops.py::TestAudioDecoderOps::test_next[asset1] - assert 0.9359999999999999 == 0.936
FAILED test/test_ops.py::TestAudioDecoderOps::test_pts[asset0] - assert 0.5760000000000001 == 0.576
FAILED test/test_ops.py::TestAudioDecoderOps::test_pts[asset1] - assert 0.9359999999999999 == 0.936

Which is probably OK. But I still find it surprising considering the implementation of av_q2d is just

static inline double av_q2d(AVRational a){
     return a.num / (double) a.den;
 }