Decoding horizontally-predicted images of more than 8bit per channel fails

[spoutn1k]

The code responsible for rebuilding a predicted row is wrong for images using more that 8 bits per channel:

image-tiff/src/decoder/mod.rs

Lines 259 to 283 in de6d9cf

	match bit_depth {
	0..=8 => {
	for i in samples..buf.len() {
	buf[i] = buf[i].wrapping_add(buf[i - samples]);
	}
	}
	9..=16 => {
	for i in (samples * 2..buf.len()).step_by(2) {
	let v = u16::from_ne_bytes(buf[i..][..2].try_into().unwrap());
	let p = u16::from_ne_bytes(buf[i - samples..][..2].try_into().unwrap());
	buf[i..][..2].copy_from_slice(&(v.wrapping_add(p)).to_ne_bytes());
	}
	}
	17..=32 => {
	for i in (samples * 4..buf.len()).step_by(4) {
	let v = u32::from_ne_bytes(buf[i..][..4].try_into().unwrap());
	let p = u32::from_ne_bytes(buf[i - samples..][..4].try_into().unwrap());
	buf[i..][..4].copy_from_slice(&(v.wrapping_add(p)).to_ne_bytes());
	}
	}
	33..=64 => {
	for i in (samples * 8..buf.len()).step_by(8) {
	let v = u64::from_ne_bytes(buf[i..][..8].try_into().unwrap());
	let p = u64::from_ne_bytes(buf[i - samples..][..8].try_into().unwrap());
	buf[i..][..8].copy_from_slice(&(v.wrapping_add(p)).to_ne_bytes());

The assignment to the variable p, which is the previous value in the chain, does not take into account the size of each sample. As we are on a u8 buffer, the previous value should be offset by sample byte count * sample size yet we only jump by sample size.

- let p = u16::from_ne_bytes(buf[i - samples..][..2].try_into().unwrap());
+ let p = u16::from_ne_bytes(buf[i - 2 * samples..][..2].try_into().unwrap());

Found this while working on #240, and pushed a fix on there.