Incorrect results during filter reversal

[Captain4LK]

The filter reversal behavior seems to be incorrect, due to the type of out, x, a, b, c being int in

rpng/src/rpng.h

Line 2141 in 07d8459

int out = 0, x = 0, a = 0, b = 0, c = 0;

Changing the type to unsigned char causes the images to be loaded correctly. According to the png spec, all the filter operations are supposed to be modulo 256.

Here I've loaded an image with rpng_load_image and then saved the output using stb_image_write, gcc 14.2.1:

The input image:

Output with unmodified rpng 1.5:

Output with type changed to unsigned char: