libavif: enable encoding support

[oreo639]

Testing the changes

• I tested the changes in this PR: briefly

Allows tests to pass for #42426

[lemmi]

Oh, so that was missing for darktable be able to export to avif. But I was wondering whether multiple encoders need to be enabled. I just glanced at the cmake file and didn't actually try and see what happens if only a single encoder is available:

-- Checking for module 'rav1e'
--   Found rav1e, version 0.6.3
-- Found rav1e: /usr/lib/librav1e.so (found version "0.6.3")
-- libavif: Codec enabled: rav1e (encode)
-- Checking for module 'SvtAv1Enc'
--   Found SvtAv1Enc, version 1.4.1
-- Found svt: /usr/lib/libSvtAv1Enc.so (found version "1.4.1")
-- libavif: Codec enabled: svt (encode)
-- Checking for module 'aom'
--   Found aom, version 3.5.0
-- Found aom: /usr/lib/libaom.so (found version "3.5.0")
-- libavif: Codec enabled: aom (encode/decode)

[oreo639]

You can specify the encoder you want to use:
https://github.com/AOMediaCodec/libavif/blob/647c3c208cf152395d777c1bf7240d2ecf7df5a9/include/avif/avif.h#L707-L715

https://github.com/AOMediaCodec/libavif/blob/f4a963865bab5edf004b670f2c9defe389087545/apps/avifenc.c#L1286-L1300

For example, the kimageformats test expect libaom and fail when comparing the image binary output to what is expected if aom is not provided.

libaom is the most important since it is necessary for lossless, enabling rav1e and libsvt-av1 allows people to use them through libavif.

[lemmi]

darktable takes a ridiculous amount of time (several minutes) to export an image. I used avifenc to see what's up and the libaom codec seemed to perform ok, while the rav1e also took considerably longer with default settings. Wasn't able to test svt, since it only works on yuv420.
So, I'm still not sure what the value of the other codecs is, but I'm happy I can enable the avif format once this is merged.

[oreo639]

You are right, rav1e is slower than libaom. My bad.