Add support for AVIF

[Wataru-404]

As title suggest, add support for AVIF?

[Woisek]

Convert into i.e. PNG and upscale that? The output will be a different format anyway. 🤷‍♂️

[NayamAmarshe]

STB, the library that we're using in Upscayl, only supports reading these formats: JPG, PNG, TGA, BMP, PSD, GIF, HDR, PIC

and writing these formats: BMP, PNG, TGA, HDR, JPG