Add support for building IO against stb_image.h from nothings/stb collection of single-file libraries

There is collection of single-file libraries https://github.com/nothings/stb which includes [stb_image.h](https://github.com/nothings/stb/blob/master/stb_image.h) offering image loading/decoding from file/memory: JPG, PNG, TGA, BMP, PSD, GIF, HDR, PIC.

An interesting dependency-less alternative that is simpler to build the GIL IO against.