Modeled from the prores version. Clips to [0;1023] and is bitexact.
Bitexactness requires to add offsets in different places compared to
prores or C, and makes the function approximately 2% slower.

For 16 frames of a DNxHD 4:2:2 10bits test sequence:

C:    60861 decicycles in idct, 1048205 runs,    371 skips
sse2: 27567 decicycles in idct, 1048216 runs,    360 skips
avx:  26272 decicycles in idct, 1048171 runs,    405 skips

The add version is not implemented, so the corresponding dsp
function is set to NULL to make it clear in a code executing it.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>