pnm: Scale samples according to specified maximum

[sophie-h]

Some formats allow to specify a maximum value for samples. Before, samples were not scaled respective to this white point, decoding some images incorrectly dimmed.

PAM (P7)

I think the tests were wrong before. But I'm not 100% confident since I haven't found something to test this format in practice.

Example

Comparison of image-rs and correct rendering.

P3 3 3 20
20 0 0
0 20 0
0 0 20
20 20 0
0 20 20
20 0 20
20 20 20
0 0 0
0 0 0

[fintelia]

Does the decoder return an error if the image sets maxval=0? If not, we need to add that or we'll end up with a divide by zero

[sophie-h]

Does the decoder return an error if the image sets maxval=0? If not, we need to add that or we'll end up with a divide by zero

Thanks for catching this. I added checks that make the decoder error earlier.

[sophie-h]

CI failures seem to be hiccups?

[fintelia]

Yep, everything passed when I tried rerunning. Thanks!