Fix the incorrect filter setting

Fix the incorrect filter setting

  - 'switch~case' in 'png_set_filter()' is dead code.
  'switch~case' updated the value of 'do_filter' using 'filters'.
  However 'do_filter' is overwritten with the 'filters' after few lines.
  So 'switch~case' became dead code, it make filter setting error.

  e.g.
  if 'filters' was #PNG_FILTER_VALUE_NONE(0x00), 'do_filter' updated
  #PNG_FILTER_NONE(0x08) after 'switch~case'. However 'do_filter' is
  overwritten with 'filters' at 1155 line. So 'do_filter' is changed
  from 0x08 to 0x00 again. It make set the incorrect filter.

Change-Id: I418ce626193f7f1225a10f6c9bdd57e102fbdb53