Allow manual TPIU packet resynchronisation

Provide the option to start processing part way through a TPIU packet. This is only relevant for TPIU or non-Bypass ITMDWT where the offset setting may need to be used to correctly synchronise the decoder.

With capture of a SWO (non-Bypass) formatted TPIU stream we would see the first TPIU data as the second half of a packet; and with no explicit way to resynchronise automatically. So the manual "user supplied" offset is a quick workaround.