x/image/tiff: add 32bit float grayscale support

[ryn1x]

Please add support for 32bit float grayscale tiff images. This is a common format in scientific and medical imaging.

[stevekuznetsov]

This would be really nice! As it is, doing image manipulations with scientific data is basically impossible with Go from what I can tell.

[rngoodner]

I have created a small package to address this shortcoming until the feature is added. It has met all my requirements so far and is written in pure Go with no external dependencies. https://pkg.go.dev/github.com/rngoodner/gtiff

[bamnet]

AFAICT there's not a great candidate to represent a 32 bit greyscale image today, I think image will need to introduce a new Gray32 type too here in addition to the decoding work.

My use case - trying to parse some GeoTIFF data. I'm very hesitant to copypasta the entire tiff decoding logic and make the few tweaks to support 32 bit images in my local codebase, but I think that's the best path given all the very clever edge cases the existing code handles.