Use reduced_still_picture_header when still_picture flag is set

[tdaede]

[rzumer]

#1498 implements necessary header changes, but we need to implement disable_frame_end_update_cdf before we can enable the reduced header in the Sequence constructor.

[cryptomilk]

I've just enabled still_picture in libavif and the encoded picture then can't be decoded by dav1d.

[barrbrain]

From AV1 Image File Format (AVIF), 2.1 AV1 Image Item:

• The AV1 Image Item Data should have its still_picture flag set to 1.
• The AV1 Image Item Data should have its reduced_still_picture_header flag set to 1.

This issue impacts producing typical sequences for libavif. Emphasis added above.

[cryptomilk]

We have added support for importing and exporting AVIF files in darktable which will be released with version 3.0.1. It would be great to have this fixed in rav1e soon.

[rzumer]

@cryptomilk Sorry for the delay in addressing this, I implemented still picture mode a few months back and it seems like other changes in the encoder broke it since then. I will add some tests to avoid future regressions and try to solve the issues found (#2112 and #2113) in a timely manner. In the meantime, #2108 should solve errors when encoding a single image frame, which I think should cover your use case.

[cryptomilk]

Awesome thanks. I'm travelling this week and will test it next week once i'm back.