Re-compress image first and display grid lines
There was a VERY weird bug where the TIFF orientation of a photo could cause the wrong coordinates to be returned.
Working around that bug by first doing a round-trip through custom canvas JPEG compression.
Also added grid to make it easier to debug.
Many prompt sessions got here, most recently this one:
https://gist.github.com/simonw/0a8b8e76cd73b1bfb72daa716641a57e