[BUG #7479] fixed scanning for start-of-frame segments in Jpeg

- the old size() implementation did a naive search for the segment markers
- which fails if segment payloads contained marker bytes
- even Fabian's old size1() was better in this respect :-)
  - but always expected length bytes (which is optimistic)
- now using a table of possible sement markers with their length bytes
[BUG #7479] removed unused code