Enabling chroma dQP does not exercise the feature as expected

[delagrangep]

When enabled, the chroma dQP index is exercised in EncCu::xCompressCU, by scanning all possible index values, depending on CU position. The local offset is stored in m_cuChromaQpOffsetIdxPlus1.
The value is set only when possible, i.e. when the CU is the first of a chroma QG. However just before checking that condition, m_cuChromaQpOffsetIdxPlus1 is always reset to zero. The consequence is a spurious reset of m_cuChromaQpOffsetIdxPlus1 for any QG that contains several CUs, defeating the purpose of the feature.
The solution is to move the m_cuChromaQpOffsetIdxPlus1=0 statement to an else branch of above-mentioned condition.

[fbossen]

​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/-/merge_requests/2018