The previous calculation code did not account for the fact that the
copy_offset for the start of the frame array is at index 0, yet the
scan line number from the rfc4175 RTP header starts at 1.
This caused 2 issues to appear:
- The first scan line was being copied into the array where the second
  scan line should be. This caused the resulting video to have a green
  line at the top of it.
- Since the packet containing the last scan line would fail the
  calculation, the packet with the RTP marker would not be processed
  which caused a log message saying "Missed previous RTP marker" to be
  outputted for each frame.
Signed-off-by: Jacob Siddall <kobe@live.com.au>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>