Add lsel (layer selection) property to progressive images.

[maryla-uc]

My understanding from the discussions in #939 and the AVIF specification is that libavif treats the absence of the 'lsel' property the same as 'lsel' being present with the value 0xFFFF, but this is done for backward compatibility only and encoders are supposed to add the 'lsel' property on layered images.

[vrabaud]

"of of"

[tongyuantongyu]

I'd like to mention that Firefox was rejecting AVIF images with 'lsel' property, so I didn't add 'lsel' property to images when I was implementing progressive encoding.

I just tested that Firefox (131.0b5) still rejects AVIF images with 'lsel' property, unless you set image.avif.compliance_strictness to 0 in about:config (because 'lsel' is an essential property but is unsupported by Firefox, so per spec the decoding fails).

Currently avifenc produces progressive images that Firefox accepts (because 'a1lx' is non-essential, so it can be ignored), but adding 'lsel' will prevent these images from being decoded even non-progressively.

I would suggest landing this change at least after we ensure major decoder implementations all have supported 'lsel'. I know the following decoders do not support 'lsel', but there may be more:

• mp4parse-rust (Firefox)
• Windows with AV1 Video Extension
  • It rejects images with only 'a1lx' as well, so this one may not count.

And even after that, this will still upset early adopters of progressive AVIFs because images will appear broken on old version of browsers, instead of losing progressive but still work.

[maryla-uc]

Thanks @tongyuantongyu for the valuable background information.
I agree with you that it's best not to add this property for now. I will add some comments in the code instead, and ideally we should work on getting this property supported in major decoders...