[Feature] Support webp images

[ddpasa]

Feature Summary

It should be possible to input webp images for image editing

Detailed Description

Maybe this is supported already but I had problems with qwen image edit with webp input. They worked after converting them to png with ffmpeg. It would be great if sdcpp could do this conversion on the fly instead of manually.

Alternatives you considered

No response

Additional context

No response

[stduhpf]

stable-diffusion.cpp uses sdb_image to load and write images, and this library only supports the following formats:

 JPEG baseline & progressive (12 bpc/arithmetic not supported, same as stock IJG lib)
 PNG 1/2/4/8/16-bit-per-channel

 TGA (not sure what subset, if a subset)
 BMP non-1bpp, non-RLE
 PSD (composited view only, no extra channels, 8/16 bit-per-channel)

 GIF (*comp always reports as 4-channel)
 HDR (radiance rgbE format)
 PIC (Softimage PIC)
 PNM (PPM and PGM binary only)

(https://github.com/nothings/stb/blob/master/stb_image.h#L23-L33)

webp is a much more complex format that involves video codecs, it's unlikely to ever be implemented in stb_image, and I don't think adding another dependency just for this is worth it.

[Green-Sky]

I would like qoi (: