Implicit partitonning issue when CtbSizeY = 128 and dual tree

[forayr]

In the specification, when CtbSizeY = 128, qtbtt_dual_tree_intra_flag = 1 and slice_type = I, the Ctb is implicitly split in a 64x64 Cb, independently of the value of the different Cb, QT, BT and TT size constraints present in the SPS and in the slice header.
Therefore, for example, the specification allows the combination qtbtt_dual_tree_intra_flag = 1 and MinCbSizeY = 128. In this case all Cb will be implicitely to 64x64 in the intra slices.
However, the VTM still call the canSplit function when doing this, which tests the different size constraints. This could lead to value of splitMode different from CU_QUAD_SPLIT at the first depth.