Download FFmpeg 5.1.1

- avformat/asfdec_o: limit recursion depth in asf_read_unknown()
- avformat/mov: Check count sums in build_open_gop_key_points()
- doc/git-howto.texi: Document commit signing
- libavcodec/8bps: Check that line lengths fit within the buffer
- avcodec/midivid: Perform lzss_uncompress() before ff_reget_buffer()
- libavformat/iff: Check for overflow in body_end calculation
- avformat/avidec: Prevent entity expansion attacks
- avcodec/h263dec: Sanity check against minimal I/P frame size
- avcodec/hevcdec: Check s->ref in the md5 path similar to hwaccel
- avcodec/mpegaudiodec_template: use unsigned shift in handle_crc()
- avformat/subviewerdec: Make read_ts() more flexible
- avcodec/mjpegdec: bayer and rct are incompatible
- MAINTAINERS: Add ED25519 key for signing my commits in the future
- avcodec/pngdec: Fix APNG_DISPOSE_OP_BACKGROUND
- avcodec/libvpx: fix assembling vp9 packets with alpha channel
- fftools/ffmpeg_opt: try to propagate the requested output channel layout
- avcodec/libsvtav1: properly initialize the flush EbBufferHeaderType struct
- configure: enable the av1_frame_split bsf for the av1 decoder
- swresample/swresample: fill the correct buffer to print the output layout string
- ffprobe: restore reporting error code for failed inputs
- ipfsgateway: Remove default gateway
- avcodec/libspeexdec: Fix use of uninitialized value
- avformat/avisynth: use ch_layout.nb_channels for channel count
- fate/lavf-image: Disable file checksums for exr tests
- tests/fate-run: Allow to skip file checksums for lavf_image
- fate/imf: Rename IMF fate-target
- avcodec/alac: don't fail if channels aren't set during init() when extradata is valid
- configure: properly require libx264 if enabled