Fix#987: Assertion on min_qp_prime_ts_minus4 failed at decoder when internalBitDepth is set to 8 and inputBitDepth is set to 10
Change min_qp_prime_ts_minus4 derivation '''(6*(internalBitDepth - inputBitDepth))''' to '''6*(internalBitDepth - 8)'''.
Change min_qp_prime_ts_minus4 derivation '''(6*(internalBitDepth - inputBitDepth))''' to '''6*(internalBitDepth - 8)'''.