Clean up and comment WPP threading code.
Clean up and comment WPP threading code.

- Remove WPP row reconstruction dependency to the row above current one in
  the previous frame. It's obviously unnecessary.
- Remove WPP row reconstruction dependency to the current row in the
  previous frame, unless the current row is the last row.

ultravideo/kvazaar,lu-zero/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar,ultravideo/kvazaar,lu-zero/kvazaar