stb_image: fix gif decoding: only restore background when dispose is 2 or 3

[cocoa-xu]

Hi, this PR should fix #1688. Background should be restored only when dispose is 2 or 3.

Minimal test code

gif-background.c

#ifdef __clang__
#define STBIDEF static inline
#endif

#define STB_IMAGE_STATIC
#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>

unsigned char* read_binary_file(const char* filename, size_t* size) {
    FILE* file = fopen(filename, "rb");
    if (file == NULL) {
        fprintf(stderr, "Error opening file: %s\n", filename);
        return NULL;
    }

    struct stat st;
    if (stat(filename, &st) != 0) {
        fprintf(stderr, "Error getting file size\n");
        fclose(file);
        return NULL;
    }

    *size = st.st_size;
    unsigned char* buffer = (unsigned char*)malloc(*size);
    if (buffer == NULL) {
        fprintf(stderr, "Error allocating memory\n");
        fclose(file);
        return NULL;
    }

    size_t bytes_read = fread(buffer, 1, *size, file);
    if (bytes_read != *size) {
        fprintf(stderr, "Error reading file: expected %zu bytes, got %zu bytes\n", *size, bytes_read);
        free(buffer);
        fclose(file);
        return NULL;
    }

    fclose(file);
    return buffer;
}

int main(int argc, const char *argv[]) {
    if (argc != 3) {
        printf("Usage: %s <gif file> <output dir>\n", argv[0]);
        exit(1);
    }

    size_t file_size;
    unsigned char* filedata = read_binary_file(argv[1], &file_size);

    int x, y, z, comp;
    int *delays = NULL;
    unsigned char *data = stbi_load_gif_from_memory(filedata, (int)file_size, &delays, &x, &y, &z, &comp, 0);

    if (data == NULL) {
        fprintf(stderr, "Error loading image\n");
        return 1;
    }

    unsigned char *start = data;
    size_t frame_size = x * y * sizeof(unsigned char) * 4;
    char * filename = (char *)malloc(1024);
    for (int i = 0; i < z; ++i) {
        bzero(filename, 1024);
        snprintf(filename, 1023, "%s/%d.png", argv[2], i);
        stbi_write_png(filename, x, y, 4, start, 0);
        start += frame_size;
    }

    free(filename);
    STBI_FREE((void *)data);
    STBI_FREE((void *)delays);
}

horse.gif

File from #1688:

Test

Assuming gif-background.c and horse.gif has been downloaded to the project root directory.

$ cd stb
$ gcc -I$(pwd) gif-background.c -o gif-background
$ mkdir -p ./output
$ gif-background ./horse.gif ./output

Visual check on the 6th frame

Before

After