* Write generation parameter exif data into output pngs.
This adds prompt, negative prompt (if nonempty) and other generation
parameters to the output file as a tEXt PNG block, in the same format as
AUTOMATIC1111 webui does.
In order to keep everything free of external library dependencies, I
have somewhat dirtily hacked this into the stb_image_write
implementation.
* Mention png text data in README.md, include "karras" in sampler text
* add Steps/Model/RNG to parameter string
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Co-authored-by: leejet <leejet714@gmail.com>
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