Detect color type of images containing pixels of signed int values

[izolyomi]

Thank you for the efforts of building this already awesome library.

I think I have a special case which is not yet covered by the image crate and I'm not sure what I could do about it.

Overall, I'm trying to load images of mixed pixel channel types where pixel values can be either unsigned or signed integer values. The crate nicely supports pixel types of unsigned integer out of the box providing the ColorType enum but lacks support for color types of signed integer pixels. I see that signed pixels are somewhat supported since 32bit floats are already present in ColorType.

How could I detect both signed and unsigned integers color types like in some other libraries, e.g. OpenCV?