Updating firstNZPos for sign data hiding

[bheng]

In CABACReader.cpp and CABACWriter.cpp, residual_coding_subblock( ) functions, it appears that the firstNZPos for determining the sign data hiding threshold needs to be updated in the final "coeff bypass" loop.

Something like the highlighted line below.

if( tcoeff )
{

int blkPos = cctx.blockPos( scanPos );
sigBlkPos[ numNonZero++ ] = blkPos;
firstNZPos = scanPos;
lastNZPos = std::max<int>( lastNZPos, scanPos );
coeff[blkPos] = tcoeff;

}

[bheng]

Merge request 755 from Spec Ticket #400 would fix this.

​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/merge_requests/755

[XiangLi]

Further fix in ​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/merge_requests/808.