Function stbi__pic_is4

lite/example/cpp_example/mge/cv/stb_image.h:6960–6967

#ifndef STBI_NO_PIC
static int stbi__pic_is4(stbi__context* s, const char* str) {
    int i;
    for (i = 0; i < 4; ++i)
        if (stbi__get8(s) != (stbi_uc)str[i])
            return 0;

    return 1;
}

static int stbi__pic_test_core(stbi__context* s) {
    int i;