Handle floating-point predictors

[kylebarron]

See https://docs.rs/tiff/latest/tiff/decoder/fn.fp_predict_f32.html and https://docs.rs/tiff/latest/tiff/decoder/fn.fp_predict_f64.html

[feefladder]

and image-rs/image-tiff#89 (comment) and the linked pdf: http://chriscox.org/TIFFTN3d1.pdf

[kylebarron]

I think this was closed by #86