The second part of this condition is intended to check whether the
current quantisation group is in the first CTU column of the current
tile.  The issue is that ctb_to_col_bd gives the x-ordinate of the first
column of the current tile *in CTUs*, while xQg gives the x-ordinate of
the quantisation group *in samples*.  Rectify this by shifting xQg by
ctb_log2_size to get xQg in CTUs before comparing.

Fixes FFVVC issues #201 and #203.