Function choose_idct_4x4_func 

pub fn choose_idct_4x4_func(
    _options: &DecoderOptions,
) -> fn(&mut [i32; 64], &mut [i16], usize)

Choose a function to implement 4x4 IDCT.

These functions get the same input but have an extra contract: Only the first 4x4 block of coefficients are non-zero. All other entries are zeroed.

The callee must uphold that contract on return