feat: write generation parameter exif data into output png (#57)

* 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>

README.md

@@ -27,6 +27,7 @@ Inference of [Stable Diffusion](https://github.com/CompVis/stable-diffusion) in
     - [`DPM++ 2M v2`](https://github.com/AUTOMATIC1111/stable-diffusion-webui/discussions/8457)
     - `DPM++ 2S a`
 - Cross-platform reproducibility (`--rng cuda`, consistent with the `stable-diffusion-webui GPU RNG`)
+- Embedds generation parameters into png output as webui-compatible text string
 - Supported platforms
     - Linux
     - Mac OS