proposal: image/jpeg: allow images larger than 65536

[alimoeeny]

Currently the jpeg.Encode() checks if the size of the image.Image passed to it is smaller than 1<<16 in either dimension.

We often work with images up to 1<<20 pixels wide.

Our current solution is to patch the go source code before build to remove this check.

This solution works fine on amd64, I don't know if this restriction is mean to protect systems with lower memory or is just a sanity check.

Proposal: modify the line lined below to allow images as large as 1<<21

https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/image/jpeg/writer.go;drc=35cbc3b55b9d5d159a5a7a160ea680b32f043bf8;l=577

[seankhliao]

cc @nigeltao

[alimoeeny]

I apologize I think I need to do more investigation, it looks like there is an actual hard limit at 65536 and going beyond it will cause some sort of banding artifact.
of course if it would be possible to fix the underlying issue and make it possible to process HUGE images, that would be fantastic.
But it looks like my proposal for just increasing the limit will not work.
Although we patch the limits, I must have accidently only tested with images that are still below the threshold.

[ALTree]

The jpeg standard does not allow images with dimensions bigger than 65535 pixels (the number-of-lines field in the header is 2 bytes).