Skip, Direct Pred Modes

* Diff from normal pred:
- Need to derive both refidx and mv. (mv only for normal cases)
-- P Skip: refidxL0 = 0
-- B Temporal: refidxL1 = 0

* Diff between B temporal and spatial modes:
- refidx:
-- temporal mode: always L0 and L1
-- spatial mode: two or one (either L0 or L1)

- mi shared
-- temporal mode: 16 4x4 blks shared one (if the colMB is 16x16 mode) or
four 4x4 blks shared one (direct_8x8_inference_flag == 1) or
no share in 16 4x4 blks.
-- spatial mode: 16 4x4 blks shared one (because the neighboring derivation is in MB boundary) or
four 4x4 blks shared one (direct_8x8_inference_flag == 1) or
no share in 16 4x4 blks.

Note:
- ColPic is a different concept from the refIdxL1.
- B picture could be the reference picture.