Allow vtag to be set

Overview

In FFmpeg we're able too set vtag for a stream, for example hvc1 is a tag of HEVC, so that we can play with QuickTime Player on a Mac (QuickTime only plays HEVC streams with the vtag hvc1, but the default vtag of libx265 is hev1).

Existing FFmpeg API

I'm not familiar with the C API of FFmpeg, I can only tell the command line option that set the vtag. ffmpeg -i input.mp4 -c:v libx265 -vtag hvc1 output.mov OR ffmpeg -i input.mp4 -c:v libx265 -tag:v hvc1 output.mov

Expected PyAV API

Like what setting pix_fmt is like. https://pyav.org/docs/stable/cookbook/numpy.html#generating-video

Example:

container = av.open('test.mov', mode='w')
stream = container.add_stream('libx265', rate=30)
stream.tag = 'hvc1'

Versions

• OS: {{ e.g. macOS 11.2.1 }}
• PyAV runtime:

PyAV v8.0.3
git origin: git@github.com:PyAV-Org/PyAV
git commit: v8.0.3
library configuration: --disable-doc --disable-static --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libwavpack --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-shared --enable-version3 --enable-zlib --prefix=/tmp/vendor
library license: GPL version 3 or later
libavcodec     58. 91.100
libavdevice    58. 10.100
libavfilter     7. 85.100
libavformat    58. 45.100
libavutil      56. 51.100
libswresample   3.  7.100
libswscale      5.  7.100

• PyAV build: Installed from pip
• FFmpeg: Installed from pip