Image scaler generates a decode error if PNG has invalid checksum

From https://gitlab.com/gitlab-com/gl-infra/production/-/issues/3243#note_473877279, we seem to be generating a hard error when there is an invalid checksum in the iCCP chunk. This chunk stores the ICC profile, which is not essential for displaying an image. Most browsers display these images without complaint.

We could strip the ICC profile out, but it's probably better to handle this in the Golang image decoder. Filed https://github.com/golang/go/issues/43382 as an upstream Golang issue.