[ticket #291] fix for context derivation of par/gtx flags
As reported in ticket #291, when sig_flag is inferred, the variables m_tmplCpDiag and m_tmplCpSum1 are not set correctly, but used for deriving the contexts for par_glag, gt1_flag, and gt3_flag.
The provided modification fixes this issue by calling cctx.sigCtxIdAbs() also when sig_flag is inferred (except for the last scanning position, where a different mechanism is used).
There is no impact on coding efficiency.