Add VapourSynth audio source filters

[msg7086]

This code is a direct translation by LLM from AviSynth interface to VapourSynth interface. As integration testing validates, the result produced by both platform are byte-for-byte identical.

[msg7086]

Path	AviSynth Comparison Result
LWLibavAudioSource, 48 kHz FLAC	Full PCM output is byte-for-byte identical.
LWLibavAudioSource, AAC with gaps and fill_agaps=1	Full PCM output is byte-for-byte identical.
LibavSMASHAudioSource / LSMASHAudioSource, AAC	The VapourSynth and AviSynth outputs are byte-for-byte identical.
LWLibavAudioSource, regular AAC at 44.1/48 kHz	VapourSynth fails on the final audio frame with failed to output an audio frame.

[msg7086]

Path	AviSynth Comparison Result
LWLibavAudioSource, 48 kHz FLAC	Full PCM output is byte-for-byte identical.
LWLibavAudioSource, AAC at 44.1 kHz	Full PCM output is byte-for-byte identical.
LWLibavAudioSource, AAC resampled to 48 kHz	Full PCM output is byte-for-byte identical.
LWLibavAudioSource, AAC with gaps and fill_agaps=1	Full PCM output is byte-for-byte identical.
LWLibavAudioSource, AAC with gaps and fill_agaps=0	Full PCM output is byte-for-byte identical.
LibavSMASHAudioSource / LSMASHAudioSource, AAC	The VapourSynth and AviSynth outputs are byte-for-byte identical.

[Asd-g]

The changes look good but there are some inherited problems:

• lwlibav audio seeking bug (not related to the agaps);
• the used POS=-1 for the dummy gap packets is valid for real audio too;
• incomplete iTunSMPB handling (AviSynth inherited problem)(LibavSmash gapless issue);
• ignored padding and duration (AviSynth inherited problem)(LibavSmash gapless issue);
• priming rescaling issue when output rate differs (AviSynth inherited problem)(LibavSmash gapless issue).

Specific behavior of the VapourSynth port - the EOF sample-count check is more strict compared to the AviSynth.

I created new more_audio branch from this codex/vapoursynth-audio-source branch. There I apply fixes for the inherited problems. I think this PR can be merged.

[msg7086]

Sounds good, merging and you can aply fixes on top of it.