M0091, part 2 of 2: QPA finalization, sub-CTU QPA for <=HD sequences, share mean-value calculations via computeAvg( )
TODO: adapt EncModeCtrl:getMaxLumaDQPDepthPos() once/if isQGStart() is modified
TODO: adapt EncModeCtrl:getMaxLumaDQPDepthPos() once/if isQGStart() is modified