Fix: Related to HashME and sequential coding

Fix for HashME and sequential encoding. It is suggested to align the sequential based encoding with the parallel based encoding by determining the hash-based ME on the first intra picture for pictures in display order until next intra picture. To enable that the enable flag in the config file is separated from an actual enabling flag based on the check on the intra picture. Then the actual enabling flag is reset to the state of the config enable flag at the intra picture.