Fix : Decoding problem for RPR case on amvpMerge

Hyeongmun Jang requested to merge hm.jang/ECM:FIX_AMVPMERGE_RPR into master

A decoding problem is observed for RPR on LDB. This issue is happened during mvp_flag() parsing with the incorrect reference picture accessing.

for the detail, After adoption of block level reference picture reordering, the reference picture index is temporally set to be 0 in refIdxLC() function even actual reference index is not 0. the correct reference picture index is derived at xDeriveCUMV() after finishing parsing process.

in the parsing stage, this incorrect index 0 is referred for mvp_flag() parsing process. if incorrect reference picture is RPR then template matching is not allowed and amvpMerge supports adaptive maximum number of mvp index based if template matching based is applied or not by JVET-Y0128.

in this case, encoder side maximum number of MVP candidate is 2 but decoder-side, the maximum number of MVP candidate is 3.

the MR fixes this problem.

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