Add .avif support (AV1 Image File Format

[Ewout]

Netflix released ​samples of the new AV1 Image File Format (​AVIF), it would be nice if support could be added for this format.

[Ewout]

Some more test files: ​https://github.com/AOMediaCodec/av1-avif/tree/master/testFiles/Microsoft

[Ewout]

Current debug commandline if .avif is used as input:

Opening an input file: C:\Users\Ewout\AVIF\Chimera-AV1-8bit-1920x1080-6736kbps-6.avif.
[NULL @ 000001c464c4a780] Opening 'C:\Users\Ewout\AVIF\Chimera-AV1-8bit-1920x1080-6736kbps-6.avif' for reading
[file @ 000001c464c4b900] Setting default whitelist 'file,crypto'
[mov,mp4,m4a,3gp,3g2,mj2 @ 000001c464c4a780] Format mov,mp4,m4a,3gp,3g2,mj2 probed with size=2048 and score=100
[mov,mp4,m4a,3gp,3g2,mj2 @ 000001c464c4a780] ISO: File Type Major Brand: mif1
    Last message repeated 1 times
[mov,mp4,m4a,3gp,3g2,mj2 @ 000001c464c4a780] moov atom not found
[AVIOContext @ 000001c464c53bc0] Statistics: 66192 bytes read, 3 seeks
C:\Users\Ewout\AVIF\Chimera-AV1-8bit-1920x1080-6736kbps-6.avif: Invalid data found when processing input

Current debug commandline if .avif is used as output:

Opening an output file: png-to-avif.avif.
[NULL @ 0000021f3f4ecc40] Unable to find a suitable output format for 'png-to-avif.avif'
png-to-avif.avif: Invalid argument
[AVIOContext @ 0000021f3f4f3b00] Statistics: 115667 bytes read, 0 seeks

[Ewout]

Depends somewhat on #7600 for lossless encoding.

Microsoft ​is going to support AVIF in Windows 10.

[cehoyos]

Definitely independent of ticket #7600.

[Ewout]

Aside from using libaom directly for encoding and muxing into an .avif file, ​libavif could also be considered.

[birdie]

AV1 decoding support is already there, the container format is probably already supported as well, why doesn't ffmpeg understand AVIF files? ;-)