Function stbi__convert_format

lite/example/cpp_example/mge/cv/stb_image.h:1792–1877

// nothing
#else
static unsigned char* stbi__convert_format(
        unsigned char* data, int img_n, int req_comp, unsigned int x, unsigned int y) {
    int i, j;
    unsigned char* good;

    if (req_comp == img_n)
        return data;
    STBI_ASSERT(req_comp >= 1 && req_comp <= 4);

    good = (unsigned char*)stbi__malloc_mad3(req_comp, x, y, 0);
    if (good == NULL) {
        STBI_FREE(data);
        return stbi__errpuc("outofmem", "Out of memory");
    }

    for (j = 0; j < (int)y; ++j) {
        unsigned char* src = data + j * x * img_n;
        unsigned char* dest = good + j * x * req_comp;

#define STBI__COMBO(a, b) ((a)*8 + (b))
#define STBI__CASE(a, b)    \
    case STBI__COMBO(a, b): \
        for (i = x - 1; i >= 0; --i, src += a, dest += b)
        // convert source image with img_n components to one with req_comp components;
        // avoid switch per pixel, so use switch per scanline and massive macros
        switch (STBI__COMBO(img_n, req_comp)) {
            STBI__CASE(1, 2) {
                dest[0] = src[0];
                dest[1] = 255;
            }
            break;
            STBI__CASE(1, 3) { dest[0] = dest[1] = dest[2] = src[0]; }
            break;
            STBI__CASE(1, 4) {
                dest[0] = dest[1] = dest[2] = src[0];
                dest[3] = 255;
            }
            break;
            STBI__CASE(2, 1) { dest[0] = src[0]; }
            break;
            STBI__CASE(2, 3) { dest[0] = dest[1] = dest[2] = src[0]; }
            break;
            STBI__CASE(2, 4) {
                dest[0] = dest[1] = dest[2] = src[0];
                dest[3] = src[1];
            }
            break;
            STBI__CASE(3, 4) {
                dest[0] = src[0];
                dest[1] = src[1];
                dest[2] = src[2];
                dest[3] = 255;
            }
            break;
            STBI__CASE(3, 1) { dest[0] = stbi__compute_y(src[0], src[1], src[2]); }
            break;
            STBI__CASE(3, 2) {
                dest[0] = stbi__compute_y(src[0], src[1], src[2]);