Refactor av1_update_neighbors

Beside above and left positions, additional above-left,
above-right, and bottom-left positions are added as
neighbor candidates.

In av1_update_neighbors, two available positions will be picked as
context neighbors.

The picking priority is
above -> left -> above-left -> above->right -> bottom->left

Change-Id: I82eaf0b23d0189caaea008ecc86776492886a05b
2 files changed