Define MergeIdxPair type to hold pair of merge indices

source/Lib/CommonLib/InterPrediction.cpp

source/Lib/CommonLib/Unit.cpp

source/Lib/CommonLib/Unit.h

source/Lib/CommonLib/UnitTools.cpp

source/Lib/CommonLib/UnitTools.h

source/Lib/CommonLib/dtrace_blockstatistics.cpp

source/Lib/DecoderLib/CABACReader.cpp

source/Lib/DecoderLib/DecCu.cpp

source/Lib/EncoderLib/CABACWriter.cpp

source/Lib/EncoderLib/EncCu.cpp

source/Lib/EncoderLib/EncCu.h