ffmpeg-full: 4.1.4 -> 4.2

Changelog: https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/refs/heads/release/4.2:/Changelog

Configuration flag changes:
https://git.ffmpeg.org/gitweb/ffmpeg.git/blobdiff/n4.1.4..n4.2:/configure

Ignored new configuration flags:
--enable-libaribb24      enable ARIB text and caption decoding via libaribb24 [no]
--enable-pocketsphinx    enable PocketSphinx, needed for asr filter [no]
--enable-cuda-nvcc       enable Nvidia CUDA compiler [no]
--disable-cuda-llvm      disable CUDA compilation using clang [autodetect]
--disable-msa2           disable MSA2 optimizations

The renamed and removed configuration options where not used.

I've added a patch to prefer libdav1d over libaom, as libaom wasn't even
able to play my sample videos for AV1 (apart from the reduced
performance, at least in theory).