texture swizzle

// pack 4x4 block
const int lookup[] = { 5, 0, 4, 2, 1, ... }; // block sample order
for (int i = 0; i < 16; ++i)
{
    dest[lookup[i]] = source[i];
}

vs.
dest[5] = source[0];
dest[0] = source[1];
dest[4] = source[2];
...