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