Release v0.20.0

• Encoder GOP/B-frame defaults (behaviour change): VideoEncoder no longer
  forces a GOP of 60 and 2 B-frames; every codec keeps its own defaults, as
  ffmpeg does without -g/-bf (libx264 keyint 250 / 3 B-frames, NVENC g
  250 with the preset's B-frames, 12 for mpeg4/mpeg2). The forced values cost
  libx264 2.6 dB at the same CRF and put B-frames into NVENC p1. Pass
  options={"g": "60", "bf": "2"} for the old behaviour. At matched
  parameters nelux now writes the same x264 options string as the ffmpeg CLI
  and encodes 1080p x264 at 478 vs 400 fps (prores_ks 477 vs 450).
• FFmpeg 9.0.2: wheels bundle TAS-FFMPEG 9.0.2 (avcodec 63, avutil 61,
  swscale 10, swresample 7); nelux.__ffmpeg_version__ is 9.0.2-tas. An
  external FFmpeg must now be a 9.x build. FFmpeg 9 makes h264_nvenc default
  to High, drops NVENC's legacy preset/rc names (use p1-p7), and changes
  MOV edit lists. On Windows the extension only loads the DLL generation it
  was built against.
• PyTorch 2.14.1 / torchvision 0.29.1 release builds; wheels keep the
  214torch tag and load on any 2.14.x.
• Audio/image APIs: merge_streams() muxes video and audio from separate
  files in-process; probe()/properties report subtitle_codecs; numbered
  PNG/JPEG sequences and 16-bit PNG output are documented and tested. CPU
  decode_image()/decode_images() decode stills in memory (JPEG/PNG from
  bytes 2.4-2.6x faster) and AudioReader is 1.6-2x faster, bit-identical.
• Fixed: concatenated MPEG-TS (cat a.ts b.ts, joined HLS segments) no longer raises "Decoding failed" at the splice when the continuity counters do not line up; MOV passthrough of Opus/Vorbis audio now transcodes to AAC with
  allow_transcode=True instead of failing at header write (0-byte file);
  NVDEC exact random-access reads could hang (lost wake-up); NVDEC
  limited-range 10/12-bit output was ~0.3% too dark; NVDEC 12-bit 4:2:0 and
  10/12-bit 4:4:4 HEVC work on FFmpeg 9; HD JPEG output uses JFIF's BT.601
  matrix; diagnose_runtime_dlls() sees delay-loaded DLLs; 16-bit gray PNG
  decode keeps exact values.