Some applications require very large images, for example medical slide imaging, where they start at 50k x 50k and routinely reach 300k x 300k. These images are often much larger than available memory and areas must be computed on demand during write.

The order in which pixels will be computed often depends on the pixel source and cannot be set by the write process. Scanline order is the most common.

To support write of images of this type, JxlEncoderAddChunkedFrame() needs to offer some guarantees about the order of data_at() calls. Just promising that ypos will always increase would be enough (this seems to be the current behaviour in v0.11, fortunately).

In discussion in #libjxl on discord it was suggested that this guarantee should perhaps be enabled with a flag setting so as not to constrain future implementations.