Typos and undefined variables in Derivation process for motion vectors and reference indices in subblock merge mode

[hanhuang]

Typos: ( xAk, yAk ) and ( xBk, yBk ) in equations 683 to 691 should be ( xNbAk, yNbAk ) and ( xNbBk, yNbBk ).

In step 4, availableA0, availableA1, availableB0, availableB1, and availableB2 are set. But availableA2 and availableB3 are not set before they are used as input in step 5.

The following should be added before step 5.

When sps_affine_enabled_flag is equal to 1, the following applies:
– The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the current luma location ( xCurr, yCurr ) set equal to ( xCb, yCb ), the neighbouring luma location ( xNbA2, yNbA2 ), checkPredModeY set equal to TRUE, and cIdx set equal to 0 as inputs, and the output is assigned to the block availability flag availableBk.
– The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the current luma location ( xCurr, yCurr ) set equal to ( xCb, yCb ), the neighbouring luma location ( xNbB3, yNbB3 ), checkPredModeY set equal to TRUE, and cIdx set equal to 0 as inputs, and the output is assigned to the block availability flag availableBk.

[jlchen]

To be fixed as suggested in the draft 8 version 2